I have run the Fatty and the Gnarly on my '99300EXC. I enjoy the fun factor of the Fatty, increases the mid for a bit more hit. The Gnarly, as advertised, increases the low end, which in the tight stuff is nice, and when racing XC or enduros, but not as exciting as the Fatty. I would still be running my Fatty but it is a bit bent :(
Both pipes make more power than the stock pipe.