Initial impressions are very good. Beautiful noise (I'm trying to work out how to get a decent recording and post a .wav file so you can hear it.
It's been a long time since I've ridden a GS with full Remus race system plus BB chip, but this feels very similar from memory. Would need to ride them back-to-back and/or do some top gear (5th, not E) roll-ons side-by-side.
Very crisp and immediate response off the throttle. Takes off from 4,000 to 6,000 revs but nothing extra above that. Increased vibration through the footpegs at 5,000 revs (could be annoying if you suffer from numb feet).
No decent backfires yet, so I'm a little disappointed I'm not scarying old ladies crossing the road in small villages.
The hanging bracket is a much neater and easier affair than the Remus system, due to the "strap" arrangement. And it's all much easier to clean than than the OE can (having had to wash it already after riding in torrential rain this morning.