IMHO: Thunderheader makes the best bolt-on pipes for the EVO. Great performance, good sound, etc.

When my stock headpipes/SuperTrapp slipon setup dies (for the second time), that's what I'll be replacing it with.

 

A 2:1 pipe will produce the most power.

I run a stainless steel Supertrapp 2:1 with 6 discs:

 

I run an EV27 cam with stock pipes (punched out). I have also ran a thunderheader on an old FXR that I had. Thunderheader is a great pipe. I loved mine. With the stock punched out pipes, air cleaner and EV27 cam I am getting over 77 HP and over 85 TQ ! I also would like to mention that she runs like a swiss watch! Very smooth,broad torque band. My only complaint is lost gas mileage (Now between 37-38 mpg). Thats around town and pushing her pretty hard...look forward to seeing what she will do on the freeway!
I have heard time and time again about the woods 6 cam. Rivercityslim loves his! You might want to research that cam as well. Good luck, hope this helps!