Love my Invidia Gemini system, no rasp at all, i do get some 'drone' as I go through 1900-2000 rpm ish (not paid quite that much attention to it), but this means at motorway speeds in 6th there isn't any drone at all, actually at 70 I can't hear the exhaust. However at 60 in 6th the revs are that bit lower and around the 'drone' area, but drop it to 5th and it's no issue. I haven't done very many miles with it as yet, maybe 200-300 so it could still be bedding in and that 'drone' may go.
I would suggest if you can get along to a meet in your area and listen to what people have installed on their motors
Mark