Frankenmike - I'm riding with the 2.35" Nevs. I don't think the mass of the tire should matter - wouldn't that just increase inertia? It'd be harder for me to speed up, but not to maintain the speed.
You're right about the inerta part, but are you taking into consideration the wide contact patch? That's a wide tire to try to go "fast" on. Lots of rolling resistance. There's also the question of your gearing. standard MTB 44/32/22? What cassette on the back?
-R