I was wondering if you where looking to shift the power band. I thought the stock pipe was good personally (great balance of high and low). I replaced mine with a FMF Gnarley and was a little disappointed with the results. I thought the little bit I gained in the low end of the power band (which wasn't much) was also sacrificed on the top end. If my old stock pipe was in better condition I'd put it back on. I sure don't want to spend more money on another pipe either.
I had both the Gnarly and the Fatty on my 99 KX 250. I thought the Gnarly would increase the bottom for woods riding, which it did, but as the KX didn't have an over-abundance of top-end it seriously hindered it there. For me it was too much in the bottom and robbed thet top. I put the Fatty on it and like was said above, it cleaned up the power band everywhere. I don't know about the Pro-Circuit pipe, but I hear good things about it as well.
I liked the performance of the fatty on all the bikes I have mounted it on. HOWEVER....on the YZ 250 the fit was terrible. Maybe the KX is better???? Make sure the shop you buy it from allows returns without a problem in case this happens to you.
I've tried the pro-circuit and it seems similar to the stock pipe but maybe some more on top end, also have a FMF fatty and gnarly using the FMF shorty silencer and definately have the best top and bottom with the fatty combo. The gnarly does give the most grunt on the bottom but does seem to sign off a little sooner on top, but is also thicker material if you are prone to getting rocks and such to dent up your pipe. I am also on my second set of FMF pipes because the header pipes cracked on both models I think because of the process of thier plating makes them brittle, I have also noticed one of my second set has a slight crack starting around a seam. FMF did send me two new pipes after sending them the old ones, and it isn't just from vibration.By the way I have a pro-circuit pipe I would sell if interested 75.00, I only used it a few times.