I think you may well have a boost leak, if the leak is after the turbo you will lose air when on boost so the mixture will be rich, however when you are cruising in a high gear, off boost, air will be drawn in and it will tend to run lean. Can you hear a whooshing or hissing on boost?
Also, you say you're seeing only 15psi at full boost, this is low for a chipped RS2 setup. Mihnea's chipset would probably run 15 psi on the spring and 25 or so psi at full throttle. The thing that senses full throttle is the TPS. You need to check this is working and you are in fact getting the full throttle signal to the ecu. The ecu then operates the Waste Gate Frequency Valve (WGFV) to turn the boost up higher than the spring would normally allow. So check the WGFV is connected and operating, you should be able to hear it click when you open the throttle hard from tickover with your head under the bonnet. On the 3B its quite easy for this not to be plugged in properly, its down at the bottom of the MAF hose near the intercooler.
Also, you say you're seeing only 15psi at full boost, this is low for a chipped RS2 setup. Mihnea's chipset would probably run 15 psi on the spring and 25 or so psi at full throttle. The thing that senses full throttle is the TPS. You need to check this is working and you are in fact getting the full throttle signal to the ecu. The ecu then operates the Waste Gate Frequency Valve (WGFV) to turn the boost up higher than the spring would normally allow. So check the WGFV is connected and operating, you should be able to hear it click when you open the throttle hard from tickover with your head under the bonnet. On the 3B its quite easy for this not to be plugged in properly, its down at the bottom of the MAF hose near the intercooler.
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