[GuidoK]

You can always fit the calipers onto the stock z4m discs.
Then you have both semi floating discs and lightweight aluminium mulitpiston calipers.


With the 370mm discs you probably need 19" rims. Also you have to check the hat depth from the new discs that that fits with the stock hat depth. Otherwise you may need to compensate with spacers or a different ET value but you risk the tyres touching the bodywork.

The 6 piston 135i m performance brakes have significantly larger pads than the ones on these 4 piston calipers.
These have pads that are 11,4cmx7,1cm
The 135i calipers have pads that are 16,5cmx7,3cm
A much longer pad so 6 pistons (to give equal pressure on the pad surface)
the 135i pads are roughly brembo 6pot GT pads.

A typical stoptech/ap racing 4 piston pad is 13cm long and typical 6 piston pads 15cm long for comparison (width is all about the same)