I have a stock pipe and an FMF Gnarly. In all honesty, I'm not sure I notice a big difference in power delivery between the two. The stocker does tuck up higher than the Gnarly though.
I swap between the two pipes. When one of them gets mangled, I install the good one and leave it on till it gets bent up. I get my pipes fixed by a local guy who uses a rig to pressurize them with water to reshape them. He also uses heat on the really deep creases. If he splits the pipes, he re-welds them. He says that the FMF pipes are more prone to splitting or developing leaks on the seams than OEM stuff.