The bike isn't even out yet, but it appears like a few European exhaust companies have gotten their hands on the CBR250R to develop some systems for them. Check it out, they all make the CBR sound like a dirtbike because of the single cylinder:
Remus (this one really makes it sound like a thumper!):
Bizumo (not sure if we get this brand in the US):
This sounds like a Chinese exhaust company. Not sure:
Which do you guys like?