I suspect most decisions are based on look, cost and weight .... but we all do things for our own reasons and that includes exhaust manufacturers I s'pose.
that's the main point. fit what you want.
i don't think the 800 makes enough power or revs high enough to really have an issue with a single muffler. just don't put bigger headers on it - people seem to love doing that and it doesn't help at all. it'll cost you power for most if not all of the rpm range ime.
we put one of our s4rs headers on to a s4rs with a remus 2-1 slip on muffler, but i didn't get to do any testing with it. one of those "part of a bike sale" deals. apparantly it went ok, but s4rs are a bit stupid in the practical power dept anyway so it's all relative i guess. i've been riding one this week with an arrow full system and u59 and the first two gears are useless. with the cam timing done it stands up so fast in second it's annoying. but i'm getting off the point now.