After being baffled by previous posts and turbology posts with differing compressor maps i've trawled the internet to try and clear up what a GT30R turbo is.
This information I'm posting is a collection of information posted global on the internet and are direct quotes.
A GT30R turbo actually refers a range of turbos.
There you go, hope it helps 
From an S2 point of view Aronmatic2 has proved you can run the GT3076 with a .63 hotside and it actually is a very good match for the I5 engine but the .82 will give you higher top end and less chance of surge.
This information I'm posting is a collection of information posted global on the internet and are direct quotes.
A GT30R turbo actually refers a range of turbos.
A GT30Rs uses three basic sizes of compressor wheel in the GT30R series , firstly 71.1mm GT35 family , secondly 76/76.2mm TO4S or GT37 families , thirdly 82mm GT40 family while all share the same 60mm turbine
In summary 71mm, 76.2mm, and 82mm compressor wheels
As Garrett has decided to name their turbos by turbine family (GT30) and compressor by major these turbo's are commly known as the 3071R, 3076R (aka GT30R!) & 3082R (aka 3040r) respectively.
Firstly the GT30R + 71.1mm GT35 compressor:
This gets called GT3071R and is sold by Garrett with the 56T compressor under CHRA or cartridge no 700177-23 . There are a couple of GT3071R "WG" or wastegated turbos around but they are inferior because of the ground down to 56.6mm turbine is fitted in bored out *******ised GT28 turbine housings .
HKS call theirs the GT2835 series and nearly always do better because of properly developed turbine housings - except those using the GT28 housing designed for SR20's .
Secondly the GT30R + 76/76.2mm TO4S compressors . Regardless of what people tell you these 7/14 blade compressors are old technology , the 6/12 blade GT family wheels kill them from a spool and pumping efficiency perspective . The S wheels are not high speed comp wheels and were designed to work with T4 turbines .
The CHRA numbers that use these wheels are 700177-10/-11/-12/-13-15/-18 . The -13 CHRA looks like it uses a 60-1 (409535-1) compressor and while it moves some air for its size is not the be all by todays standards .
The GT30R + 76.2mm GT37 compressors are very good things . Garrett Japan make them in 48 , 52 and 56 trims which are marketed by HKS as GT3037 48/52/56 . Garrett also market the 56 trim version which they often call "GT30R".
Generally the only way to identify any GT BB cartridge is to look at the ID tag's CHRA number which for GT30BB will be 700177 series . For these what I call propper or real GT30R's the CHRA numbers for 56 trim is 700177-7 , for the 52 trim is 700177-6 . I'm reasonably sure the 48 trim version is the 700177-5 , its not common though now I believe available through HKS for the Subaru WRX.
The Garrett catalog shows the real GT30R 56T as unit no 700382-12 without turbine housing but you get to option one of three available turbine housing kits.
The GT30R + 82mm GT40 compressor .
These are usually called GT3040R or more recently GT3082R because Garrett has decided to name their turbos by turbine family (GT30) and compressor by major diametre (82mm) . The "R" means rolling element (read annular contact) ball bearing centre section.
Both Garrett and HKS market slightly different versions of the GT3082R .
Firstly HKS's version uses a 50 trim GT40 BCCW-18 compressor .
Secondly Garretts uses a 56 trim GT40 BCCW-18 compressor .
Both compressor choises are really overkill for the 60mm turbine , its all fine and well to shove air into an engine but the exhaust escapes via the turbine/housing so if mismatched banks up before the turbine . The HKS's 50T version suffers less because its smaller inducer lowers the wheels pumping capacity compared to the 56T Garrett marketed version .
The CHRA numbers are 700177-9 for the 50T and 700177-14 for the 56T .
If you compare exhaust manifold to inlet manifold pressure under boost the results show this combination to be a poor performer .
IMO better options are the GT37 compressors in 52 or 56 trim depending on what airflow is required.
Also IMO turbos should be sized to give the power required now , not with room to grow because doing it that way WILL buy boost lag now . The turbine power to drive large compressors is not free so if under utilised the wheel WILL cost you response .
Turbine housing AR's . We should really look at turbine maps and compare them to the map of the compressor we decide to use .
Small AR turbine housings are a bad read unsuccessful way to excite large compressors/turbos.
The ratio variable is there to control the exhaust gas speed into the turbine blades , believe it or not small housings often cause more rotating group lag than a propper sized one .
IMO the .63AR GT30 housing is only any good for the GT3071R or the 48 trim GT3037 . Once you get to the 52T GT3037 or 56T GT3037/GT30R its really going to need a .82AR or even the largest 1.06AR for the nth in top end .
In summary 71mm, 76.2mm, and 82mm compressor wheels
As Garrett has decided to name their turbos by turbine family (GT30) and compressor by major these turbo's are commly known as the 3071R, 3076R (aka GT30R!) & 3082R (aka 3040r) respectively.
Firstly the GT30R + 71.1mm GT35 compressor:
This gets called GT3071R and is sold by Garrett with the 56T compressor under CHRA or cartridge no 700177-23 . There are a couple of GT3071R "WG" or wastegated turbos around but they are inferior because of the ground down to 56.6mm turbine is fitted in bored out *******ised GT28 turbine housings .
HKS call theirs the GT2835 series and nearly always do better because of properly developed turbine housings - except those using the GT28 housing designed for SR20's .
Secondly the GT30R + 76/76.2mm TO4S compressors . Regardless of what people tell you these 7/14 blade compressors are old technology , the 6/12 blade GT family wheels kill them from a spool and pumping efficiency perspective . The S wheels are not high speed comp wheels and were designed to work with T4 turbines .
The CHRA numbers that use these wheels are 700177-10/-11/-12/-13-15/-18 . The -13 CHRA looks like it uses a 60-1 (409535-1) compressor and while it moves some air for its size is not the be all by todays standards .
The GT30R + 76.2mm GT37 compressors are very good things . Garrett Japan make them in 48 , 52 and 56 trims which are marketed by HKS as GT3037 48/52/56 . Garrett also market the 56 trim version which they often call "GT30R".
Generally the only way to identify any GT BB cartridge is to look at the ID tag's CHRA number which for GT30BB will be 700177 series . For these what I call propper or real GT30R's the CHRA numbers for 56 trim is 700177-7 , for the 52 trim is 700177-6 . I'm reasonably sure the 48 trim version is the 700177-5 , its not common though now I believe available through HKS for the Subaru WRX.
The Garrett catalog shows the real GT30R 56T as unit no 700382-12 without turbine housing but you get to option one of three available turbine housing kits.
The GT30R + 82mm GT40 compressor .
These are usually called GT3040R or more recently GT3082R because Garrett has decided to name their turbos by turbine family (GT30) and compressor by major diametre (82mm) . The "R" means rolling element (read annular contact) ball bearing centre section.
Both Garrett and HKS market slightly different versions of the GT3082R .
Firstly HKS's version uses a 50 trim GT40 BCCW-18 compressor .
Secondly Garretts uses a 56 trim GT40 BCCW-18 compressor .
Both compressor choises are really overkill for the 60mm turbine , its all fine and well to shove air into an engine but the exhaust escapes via the turbine/housing so if mismatched banks up before the turbine . The HKS's 50T version suffers less because its smaller inducer lowers the wheels pumping capacity compared to the 56T Garrett marketed version .
The CHRA numbers are 700177-9 for the 50T and 700177-14 for the 56T .
If you compare exhaust manifold to inlet manifold pressure under boost the results show this combination to be a poor performer .
IMO better options are the GT37 compressors in 52 or 56 trim depending on what airflow is required.
Also IMO turbos should be sized to give the power required now , not with room to grow because doing it that way WILL buy boost lag now . The turbine power to drive large compressors is not free so if under utilised the wheel WILL cost you response .
Turbine housing AR's . We should really look at turbine maps and compare them to the map of the compressor we decide to use .
Small AR turbine housings are a bad read unsuccessful way to excite large compressors/turbos.
The ratio variable is there to control the exhaust gas speed into the turbine blades , believe it or not small housings often cause more rotating group lag than a propper sized one .
IMO the .63AR GT30 housing is only any good for the GT3071R or the 48 trim GT3037 . Once you get to the 52T GT3037 or 56T GT3037/GT30R its really going to need a .82AR or even the largest 1.06AR for the nth in top end .
From an S2 point of view Aronmatic2 has proved you can run the GT3076 with a .63 hotside and it actually is a very good match for the I5 engine but the .82 will give you higher top end and less chance of surge.
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