I recently put an e-series on. I originally was going to go with the Megamax but after doing extensive research I found the E-series consistantly ranked highest in after market exhaust overall. After putting it on, it has a good sound and noticable low end throttle response. As for solid horsepower numbers I can't say but in most magazine it usually will get you at least a couple extra horsepower.
2001 400EX
White Bros. E-Series Slip-On
Jetted (155 Main, 40 Pilot)
Maier Woods Pro Aluminum Handguards
AC Racing Bumper w/ Mudscreen
Cohoona Neoprene Shockwears
OMF Aluminum Scoops
Maier Oil Line Guard/Frame Guard
Scream'n P-Brake Block Off
(SOLD)