I dont have an estimate yet, but I can't imagine it being enough. This guy has been making exhaust for several years. I can tell you - my shorty 2bros gsxr pipe i use to run when i first got it - didn't have enough back pressure. So I'll know what it feels like! haha.
fmf headers should work it only makes sense. if a moto works exhast fits on a stock header and an fmf slip on does the same thing, and u can add an fmf header to an fmf slip on then whats the prob. just get an fmf header there very nice