Is it a 2001 XR80, or an earlier one?
If it's earlier, then I'd look at Powroll
www.powroll.com and Supertrapp (ask your local dealer to let you look at one in their Parts Unlimited or Tucker Rocky book) before you make a decision.
The disk style muffler (like the Supertrapp & Powroll systems) seems to work best on the small XR's.
If you have a 2001 XR80, then a pipe won't do as much good as on the earlier models because the stock pipe is better to begin with!
On an earlier model, expect about 8 - 15% increase in horsepower, depending on which pipe you buy. It'll be better throttle response and more top end.
BTW, the sound will be increased considerably from stock, but wouldn't be lound by big bike standards.