The dyno numbers I quoted are a consensus of numbers compiled from reports I've seen, so they should be representative.
Would you think that the dynos are even comparable since they were probably done years apart, and maybe even on different dynos? I'm not really concerned about wide open throttle, since I'm almost never there on the 450....I would wager to guess the 03 makes slightly more power where the 08 exhaust gives up a bunch of hp due to its restrictive design. Either way, my 03 is long gone, but I have some fond memories of shifting once per lap on that thing.
The '08 exhaust is restrictive, but once that's taken care of the bike is a beast. Either the '06 or '08 can be ridden in one or two gears around a lot of MX courses, if that's your style. The '06 actually pulls better at very low revs than the '03 (I've owned both at once), and revs much harder on top. The fact is that the '03 is simply so much work to manage that it tires the rider more than the newer bikes, and the '06+ engines can and do produce better traction and lower lap times because of the lack of wasted time in wheel spin.