will a big-gun or white bros headpipe fit on a stock 300ex silencer?if so what jet size do you think ill need.?what kind of performence

 

If a new header is the only upgrade you do, I'd be surprised if you would need to rejet at all. However the only sure way to know is to do a plug read after test riding.A bigger header is a good idea on the 300ex, but the increased air flow is still being trapped by the restriction in the stock pipe/silencer