i've got the dsp stainless unit on my '00 426. love it. i believe i have the motocross header, and it runs a lot like my '99 400 with the fmf silencer and power bomb. the 426 hits harder and makes more power to be sure, but the style of delivery is very similar. it "breathes" easily, if that makes any sense. to compare, my buddy has the same bike, and for a while he was using the white brothers e-series. the bottom end was monstrous, but the bike felt stuffed up - maybe it was a difference in compression braking.
anyhow, my elevation is around 1000' and i'm jetted with a 168/45 pilot/main combo.