I just installed StopTech sport slotted (Cyro-treated) rotors and hawk 5.0 pads all around, along with a full brake flush. I can't tell you how massive the difference is from the rusty pieces of shit and crappy pads my x4 35i came with.
I found the OEM brakes on the x3/x4 to be very susceptible to fade under repeated use (high speed, or aggressive braking on a steep incline), the stop tech rotors are much more durable with the cyro-treatment and the slots really do help with heat endurance. The slots also help improve the initial bite, and help prevenr pad glazing. The hawk 5.0 pads are wonderful, very "comfortable", predictable gradual brake bite, high thermal endurance and very very solid in HEAVY rain (I guess the slots helped with that as well).
I can now stop from 90-110mph to 60, VERY aggresively without any fade on repeated attempts. We'll see how this holds up over time as the pads and rotors wear down, but for now they're amazing.
Benefit to cost ratio! I'm very satisfied, and given I'm not tracking, just driving HARD (and I mean that)... they work great!
The rotors aren't the biggest out front, but given the rear bias I think most people are dissatisfied when first driving these cars due to how the DSC module distributes brake force to the rear axle first (supposedly to prevent body lean). Bigger brakes wont help with that, unless the brake coding can be enabled on the DSC module like the F30, which shifts brake force priority to the front.
http://f30.bimmerpost.com/forums/sho...d.php?t=764547