I have rode one with a FMF powerbomb complete system-the owner said it allowed the bike to pull better throughout the powerband and seemed to make the power broader,he didnt seem to be hitting the rev limiter as much with the fmf system.I never rode it std so cant compare.If you want the info direct speak to andy at www.1stmx.co.uk
I also read a high rating on the FMF moto powerbomb with the ti 4 muffler. Get the ti 4 " ama legal " ( 3 inches longer ) if noise is an issue. You do lose a little performance with the ama legal ti 4 tho. According the dirtrider test, tho, this combo gave 2.5 to 2 extra HP. almost all the way through the powerband with no loss on the very top.:)