You can fit 21 exhausts but the "stingers" do come off at a slightly different angle so the silencer mounts wont line up. although this wont be an issue if you are using aftermarket silencers.

I totally get where you are coming from with the stock look but I would go with a set of aftermarket pipes, they just sound so good you definetely wont regret it and make going under bridges and through tunnels a very enjoyable experience

The choice of pipes is down to personal preference but as mentioned before Tygas offer excellent quality and value and are readily available but if you want to go for something a bit more exclusive keep an eye out on yahoo auctions for a jap market set. My personal favourites are the JHA 500V replica , I know someone who has a pair hanging in his shed but he wont sell them not that I can blame him for that.

With regards to the bodywork clearance issues I painted up another r/h lower panel with a bulge to clear the pipe and hammered a small flat on the l/h pipe to clear the l/h panel to clear the Tyga pipes on my mc21. You can now purchase some good quality aftermarket mc28 rothmans decal kits so painting up a second hand panel shouldnt be a big drama.