Great bike! I had a W650 for a while and really loved it. There were some web sites out there about the W650 that had links and information about different accessories for them. As you already know, they didn't sell many of these in the U.S. so aftermarket parts support is pretty slim. However, I think some of the bigger accessories guys, like Jardine, may make pipes for this bike.
Personally, I thought the stock mufflers on the W sounded really good... as the bike got more miles they tended to get a bit louder but never harsh. Definitely stay away from full-on open drag pipes, you'll mess up the power curve of the motor and they are just too loud and obnoxious in my opinion.
The other option is to keep an eye out for some stock W650 mufflers on eBay and burn out some of the baffles. Still stock but a bit louder.