I'd say that the first thing you need to do is to determine where you want to make the power improvement. I know of no pipe that surpasses all other pipes thruout the band. If you want top end, for example, a tunable disc pipe will do a good job by adding more discs than it comes with. The White Bros. E series is one of them as is the Super Trapp.
If the low to mid range is where you want more power, The Pro Circuit T4 works well in that area..at least it does on my Raptor. I have not heard a lot of good things about Big Gun or FMF either.
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I read a article on pipes for the EX and the very best pipe gave a whopping 2 horse gain, If you want a big gain you will have to put a cam, bigger piston, pipe, air filter, jets, then dial it in, in other words get a 410 or 440 kit