[RoundelM3]

Flinchy - you're not going to get any kind of significant power increase with a cat-back exhaust on your 128i. Cat-back or axle-back, the most you'll see is 4-5 hp at very high rpm. If you want a power increase you can feel, get a set of power pulleys installed - you will definitely feel that, and an axle-back exhaust will add to the fun.

As for cost, I bought my BMW PE for the 128 for around $700. And again, this is a BMW part that you won't get any hassles about. If you remove your catalyst and entire exhaust system, you may get flak from the dealer and you will also be tampering with federally-mandated pollution control equipment. If you really want a lot of power, trade your 128 in on a 135. You can put everything available on a 128i (short of a supercharger kit) and you still won't approach the same power level - not by a long shot. I had a 330i ZHP that I added an intake, exhaust, power pulleys, a larger throttle body and Dinan's S3 software for that engine, and I still didn't come close to 300 hp. The ZHP engine had 235HP stock, with higher-performance camshafts, and there just wasn't much more you could get out of it - maybe 20hp with everything mentioned here.