I'm on pipe number three on my bike, and looking for comments from other members. After I crushed the stock pipe, I got an FMF Gnarly (not the desert), and found it really added to the bottom end power, although it went flat on top a little earlier. For motocross, I thought it was an improvement on stock; maybe for hill climbing or wide open stuff stock would be better.
After I got landed on in a race (another story) I cartwheeled down the other side of a tabletop and really banged up the Gnarly (which is hard to do, because it was way, way stronger than stock and really dent resistant). I then moved to a Zip-Ty, which fit beautifully, had a better mounting system, and looks cool. I seems strong, and has less bottom than the Gnarly, but more on top. I'm not sure how the Gnarly Desert would compare to the Zip-Ty, so if others have feedback, I'd love to hear it.
I managed to get some of Destry Abbot's used stuff and spares last year, so I now have a brand new, undented Gnarly (I just need someone to match up the old one with the undented version). I've also got an FMF Silencer with a Cobra Sparky welded on, and I'm really impressed with having a forestry improved spark arrestor with no noticible power loss.