Re: BA Auto. is it slipping or is this normal?
Sounds normal to me, but I would still get it checked.
Having a larger exhaust put on will reduce back pressure, some larger exhausts are fitted with a turbo muffler to introduce back pressure again, therefore eliminating the reason for having a larger exhaust put on in the first place.
The advantages are a lower idle in top gear and longer rev range, this is what creates addiional top speed. And will change your exhaust note, depending on the type of extractors used.