Check out the winter issue of Snowtech magazine. It has all the breakdowns on the SLP single and twin pipes for the 2002 polaris 800 twin.
They claim 149.9 HP @7900 rpm with the single pipe (vs. 136.9 HP @7800 rpm stock) This also includes a high flow SLP airbox mod kid, $17.95.
Twin pipes claim 157.3 HP @ 7900 rpm. The only drawback to the twin pipes is they increase the compression so much you either have to replace the stock head or use race gas/octane boost every fill up.
Hope this helps.