Dear member, as said many times, I replaced the stock kat with a 200 cells by Magnaflow on my Mini Cooper S.
And performance and power increased a lot! (something like 10-15 HP)

Often I imagine how much power I would have on the 545i replacing the two kats.

It would be a cheeeaaapp mod (Click, EUR 250 for parts and 300 for the job) and the car would gain something like 20-30 HP.

+10% more power and lower consumption.

I'd like to do it, but I know the car would become noisier.

I would be faster than 550i and imho fast as the E92 M3 (on straight - M3 has a little V8 and long gears)

I would like, but I cannot