[uberschnell]

Here is a photo of the SS pistons up close. It's a cheap upgrade from the aluminum and well worth it. (Brembo, of course, are already SS)

Without getting into the specifics comparing stock vs R1, piston's are 36mm, 32mm, 30mm, and 28mm. I suspect this will move the brake bias forward, but not by a tremendous amount with the stock 135i rear calipers.

As you can see on the Brembo brakes, the pistons are much beefier. In fact, I'm not even sure where the dust shields are on the R1 brakes. It's clear the Brembo would probably be better suited to track duty, but the reality is I am comparing $3800 BBK to a much more reasonably priced alternative, so something has to give.