To be honest I want the rears bigger for the aesthetic, I think 380s on the front and 310s on the back looks silly. I'm happy to keep the rear calipers the same, as most of the breaking is done by the fronts.
310x22 is the biggest MQB rear brake I can see used by R/
Cupra/RS3/RSQ3 etc.
To go bigger your looking at something like an RS4 at 330x22 or and RS6 rear at 356x22 but even the 2+ tonne turbo V8 RS6 has a single piston sliding caliper - similar to your caliper just a little bigger. No idea if these would fit/bolt on?
As I mentioned in your IM, one of the difficulties with going 4 pot rear caliper is loosing the handbrake function.
To go to a 4x piston rear brake your looking at an Audi R8 setup!! with 356x32 rear discs and a separate handbrake caliper.
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Some BMW's and AMG's have 4 piston rear brake but the use a small drum/shoe in hat hand brake design - and would need the whole rear hub/bearing/suspension modifying.
Theoretically possible - but certainly very expensive, and these cars with big rear brakes also have massive rear tyres to make the most of these big brakes vs 225/235!
Just bolt on the
SUB8 front brakes or the Porsche Macan caliper mentioned above - either of which you'd need to check your wheel clearance.