I have a 2001 XC 800 Sp with an SLP single pipe and can, gutted air box, goodwin clutch kit, and v force reeds and I have a chance to purchase a set of PSI cheater heads. What do you guys think, should I buy them, what kind of hp increase?

First off you have a slp pipe so if anything you would want slp heads that have been tested together with your pipe.

Chances of anything that psi makes being any good are slim these days.

Usually you lower compression when installing a pipe so you loose hp, but the design of the head angles, squish band, will increase hp slightly.

So you might get 1-3 hp more with slp heads, psi heads,,,, probably make a good ashtray.

Basically a waste of money if you ask me.