I think it's just common sense that on a 528mhz processor, 2.1 will not run as smoothly. To my knowledge the Hero does not have a separate video processor like the iPhone GS or the Pre. Most likely I predict that we'll have to turn off any fancy features in 2.1 to avoid lag. But again, that's just my personal guess.
Yeah, Live wallpapers don't seem like anything special. Sure, it's cool, but nothing I would lose sleep over if HTC doesn't include it in the Hero 2.1 update.