Ok, here is the scoop:
135i rear caliper weighs 5 lbs (vs. my original Z4 3.0si at 6.6).
Pads are roughly the same weight.
Net net, the weight difference between my stock setup and this updated setup is: 0 lbs!
Now, rear piston diameter is 42mm (stock is 40), also larger rotor and more pad material, definitely increases rear bias. As I said previously, the front was also increased by larger rotor and larger pad (albeit same piston up front), thus I shouldn't be too far off from original bias.
Caliper just clears the 328mm rotor. Pads just clear the inner diameter. I'll have to create proper spacers for the caliper (using mock up for now). Welded the rear z4m brake shoes to the z4 3.0si shoes, and tested brake application. Works.
