Sorry I didn't post these until now. We checked our pads early in Nov before the EuroTuner event and the pistons look just fine. The dust boots have melted a tiny bit, but that is completely normal. We had a total of ~5 track days on the pads using Motul fluid.
We had a chance to speak with Brembo in Nov and we both agree that it is NOT the calipers or ceramic cups at fault here. It is the donor pads which are causing the ceramic cups to crack.
As you can see there are the gas reliefs built into the backing plate of the pad. The pistons are supposed to line up correctly with these pads. You can see where the piston is half over the backing plate and half over the relief cutout.
The solution would be to either get a set of custom backing plates cut, or wait for the manufacturer of these pads to get an actual 135i backing pad made. The pads that we have (and I'm sure the same pads that Ralph's car has) are originally from a Porsche, and are then cut down to fit our 135i calipers.
So we'll see more as people start to track their cars and check their pistons, but I think that this isn't a piston problem.