[Obioban]

I'd rather have the stock 128 brakes than the stock 135 brakes-- no need to feel cracking pistons or melting dust boots.

The purpose of more pistons is to have a larger pad, which gives you longer pad life-- NOT to apply more force. The stock 128i brakes can lock the wheels (every time you engage ABS that's what's happened), so there is zero benefit to applying more force. The benefits of BBKs are:
More heat capacity
better pedal modulation
longer pad/rotor life (largely due to the increased heat capacity)
reduced unsprung/rotating mass
pad swaps without removing the caliper from the car (useful if tracking and changing multiple times per weekend).

The 135/performance kit may have increased heat capacity, but it's useless because heat cracks the pistons... so if you use it for anything serious, where you'd need the capacity, it's a terrible choice. It doesn't come with stainless lines, so you still don't have good feel/modulation. It doesn't have lighter rotors, so you're not decreasing rotating mass. And, I'm not positive, but I'm fairly certain you still have to remove the caliper to change pads, so you don't gain that.

I completely agree with Stig-- I'd stick with stock or go to a real BBK. The BMW performance kit/135 setup is worse than the stock 128 setup, imo.