Nisfan - The B63 is designed for the standard exhaust and the B63S is designed for sports cats. I used Shell V Power on both occasions (and 90% of all other times too). Below is the dyno chart showing stock (blue line) and Brabus (red line). As you'll see, the extra 4bhp comes right at the...
As manufacturers turn out ever improved drivetrains, it's getting harder and harder for tuners to make improvements because the engineering margins they work in are getting smaller and smaller.