I have K&N air filters.
the noise gives a marginally increase in Induction roar, but only when you open the throttle hard.
No exhaust noise increase.

I removed my rear silencer unit, opened it up & chopped out all the tubes and baffles inside, re-welded it together, and WOW, nice little rasp from it now....
The note, is still SILENT at a constant speed, but if you cog down a gear and open the tap, she roars nicely, but not enough to upset all your neighbours....

Just give a nice rasp note and you hear your Xfire grizzle nicely..
The performance it changed marginally, I had a tinsey bit less torque at lower revvs, but power ALL the way to 6,000 revvs. The power comes in, in abundance at about 2,200 revvs and stays there all the way.

Sounds nice too, and is a very cheap hop-up, and won't cost hundreds of $$$$ for a new back box.

there is an article on this site with pictures, but if you want to mail me your private mail address, I will gladly mail you sketches of exactly what you need to do to the silencer.

Grant B