I have a buddy with an 05 400ex. His is stock, mine has a pipe, jetted, airbox opened up, and dropped a tooth on the front sprocket. We've both been riding well over ten years, so it's not like either of us suck. I can walk away from him in the woods on any given day, and he can do the same to me. I'd say they're pretty evenly matched. He does have a few more mph up top, but as tight as the trails are around here, you're never in sixth gear opened up anyways. If you can ride, I really don't think a 400ex is that much of a challenge. If you're not that good of a rider, a decent rider on the 400 will kill you.