another brake improving mod I've always had on my lengthy "to do" list is swapping out the rear solid REAR rotors of the 325/328i/is for that of the 328iC models which are fitted with thicker vented rotors and slightly larger diameter pistons in the calipers which will improve braking moderately but are more cost effective than other options.
I think it would be a nice match with the upgraded front M3 brake rotors, calipers, spindles etc. as mentioned above. It saves you some $$$ from having to replace rear trailing arms and such to do the M3 brake swap in the rear and will help over the stock rear brakes. which would be a less than ideal combination with M3 front brakes.
i priced it out a few years back and I'm pretty sure it came in around $300 to do the rotors/calipers/pads with aftermarket replacement parts.
just something to think about.