My roomate has a 675 and just recently put on a two bros. exhaust. Huge increase in volume compared to stock. I haven't ridden it since, but he says that there's a slight, but noticeable power increase after he got some tuning done. He originally wanted a Jardin, but his brother hooked him up with this exhaust for dirt cheap.
Two Bros definitely for an underseat exhaust. I haven't seen better. BTW the best "mod" for your bike is a motion pro black helmet holder,I mounted my cousin's to his subframe. It's unobtrusive and he really likes not having to carry his lid everywhere!!!