For those that may be interested, I've upgraded my X3's rear brakes from the stock rear 330 x 20 mm rotor with a 42mm piston caliper setup to an F10 (2009-2016) BMW 550i setup, which is 345 x 24mm, with a 44mm piston. This setup has more thermal mass in the rotors and 10% more braking force just from the increase in piston size. There's a small increase in brake torque from the slightly larger diameter rotor. The pads are larger, which should help longevity. I was replacing rear pads every 20,000 miles or less. I feel these are a good match to the X5 50i 365 mm front brakes that I'm running. It's essentially, functionally and technically identical to the 345mm rear setup on an X5 50i, just with parts from the sedan so that we can use the electronic parking brake and have a rotor offset that will work. So, my X3 now has the equivalent of X5 50i brakes all around!
I really like the 550i brakes in the rear. It feels slightly more rear biased than before. Modulation seems markedly improved. The biggest benefit seems to come in terms of handling. The braking balance is restored back to factory. Not that I even noticed an issue with it before. The car seems to suffer less from brake dive. Overall braking force seems increased. Definitely a lighter pedal input for the same braking effort. Turn in while trail braking is way better. That part alone makes me very happy that I did the rears. If they last longer, all the better.
I'm using F10 550i rear calipers, rotors and pads. The stock X3 brake lines and pad sensor works fine. The parking brake actuator and hose bracket transfer over to the 550i calipers easily. The spacers I've made allow for lateral offset correction. One edge of the heat shield needs to be trimmed or bent back slightly. One important thing to note is that the calipers on the 550i are trailing mount, behind the axle, rather than the X3's leading mount. The difference is that the calipers mount on the opposite side when fitted to the X3. So, right on left side and vice versa for the entire caliper and bracket assemblies.
The rotors are OEM part number 34216775289. The brake caliper and bracket part numbers are listed below, as well as a list of donor vehicles that they were found on. Used calipers go for around $60 per side. Remanufactured calipers are closer to $150 per side or more.
I ordered a small batch of spacers. Enough for 10 sets. Here's the ebay link if you want a set:
BMW F25 X3 F26 X4 brake upgrade adapter kit for 2009-1016 BMW 550i rear brakes PM me for a bundle discount on front and rear X3/X4 brake spacer upgrades.
Here are a few pictures of the results.