Yea Idnan this is what I thought, As I was led to believe that the Calipers were the same as the ones on the 135i albeit the one'ers ones are a gray ish colour.
Apparently this theory is thrown out of the window because accornding to member over on M3cutters, who are currently in a group buy for this front set up and saying that the 135i caliper pistons are prone to cracking. Where as the BMW Performance uses different pistons and can withstand more abuse.
The members run the set up on track use and they come recommended.
In regards to disc size I do not know im afraid. as I would be running the Caliper with CSL discs which may well be smaller than the 335 set up.
I was actually considering buying this for myself for my ol' bruiser.