I've been using my local dealers demo bike for the last week, while my GS was in getting some warranty work done on it. The demo bike was fitted with a "full" Remus system. (Headers and "silencer")
Whilst I enjoyed the sound to begin with, I found that it was quite annoying after a couple of long trips (Edinburgh to Aberdeen). The noise was that intrusive that my ears were "ringing" for a good half hour after getting off the bike.
IMHO the Remus system looks better than standard, and made the bike feel as if it was reving more freely. However I don't think that it made the bike any quicker.