Both the Pro Circuit and FMF are much lighter and reportedly add about 1-1.5hp w/ proper jetting. The pilot will need to be richened up to a 38 and main to a 105 @ lower elevations. Other than those two adjustments, you can only adjust idle speed, needle clip position and air screw...all of which should remain stock.
I've got the ProCircuit and it is a nice fit w/ a decently quiet exhaust note.
One of the best pipes out there is one of the least known. Check out Frank Nye at Engines Only http://www.xr100.com/xrparts.htm and take a look at his Outlaw Series XR100 pipe before you make a decision. This is a proper pipe design , unlike a lot of the pipes coming out some of the west coast shops.