Hmm. I thought there was trouble even fitting the RB 135i rotors (345 x 28) in the 135i calipers if one is to use Ti shims? Here's the data I have gathered
135i OEM: 338 x 26
135i RB: 338 x 28
E46 MCP/Z4M OEM: 345 x 28
E9X M3 OEM: 360 x 30
So it'd sound to me that if one wants to run shims, the max is ~28mm rotors. If one is to go for 30mm rotors, at the very minimum the shims would need to be given up? I guess with massive enough rotors, we trust that heat dissipation would be sufficient that the shims would no longer be needed? Otherwise, I much rather to have some brake fade (by going with a smaller rotor but with shims) than to need a caliper rebuild...