I'm not quite clear on this insistence of yours regarding the 5555?
I presume they are using the 5575 and 355mm discs because they will fit behind an 18" wheel (albeit not the standard one). I presume that with larger discs you will need a 19" wheel which means larger and more expensive tyres, more mass to accelerate, possible adverse effect on handling and with a small car like the 135i perhaps 19" wheels are a little out of proportion.
But what do I know? I'm leaving it to AP and Birds to sort out - I imagine they know what they're doing.