Dolph, my experience with cat removal is with a car, your mileage may vary, but all cats work the same, so there are parallels that might be worth considering before you get the screwdrivers out.
I found no appreciable change in any measure of performance...either instrumented or seat of the pants. What was noticeable...and really noticeable...was the amount of smoke and filth that came out of the pipes under acceleration..so much so that the smell was quite noticeable. Likewise, the stuff that the cat otherwise would have burned off stained the back of the car.
Would it happen in a M3W? Don't know, but again, the function of the cat remains the same regardless, so it's removal does lead to an increase in stuff coming out. What I discovered is that it's removal didn't add anything beneficial to performance or the driving experience, so I installed a low restriction/high flow cat...and left the cat on my M3W and wouldn't even consider it's removal.
