Try not to mess too much. If stock is good enough for the M5's S38, its good enough for your motor. These cars have very decent tailor-made exhausts, not shop-constructed 'this should be good' type pipes that aren't engineered, tested, tested and tested again. The only ones that get close are Supersprint and Eisenmann. Remus are not better than stock- just noisier. Most will reduce economy, but since empirical test data is not available on any of this understanding the finer points is very hard.
btw; resonator on the 3.5 is a must. Without it you and your occupants will be driving (or more likley be driven) to an asylum in no time flat.
Sorry for the bad news, but its the way it is. Don't take it so hard though, the stock setup is really very good. Only trouble is rust when beyond 10 years old.Nick