Guys who have done the Ur-S6 rear brake....

[panzerwagen]

caliper and disc upgrade. How big of a disc are you using? I'm thinking 280mm on mine but wonder what is the minimum size disc I can use with this upgrade? 275mm? 270mm? I'm thinking of having Compbrake make a custon rotor and disc for me. Opinions? I'm trying to avoid having to install a Tilton brake proportion valve or something to that effect because the disc may throw the balance off.
Oh and my front discs are 309mm! with Boxster Brembos.