How would these wheels do for a lighter duty downhill wheelsets?

These guys are dead wrong... those are fully capable downhill wheels. The 823 is actually the most 'accomplished' downhill rim ever, more podiums and top finishes than any other rim. Personally, I'd go with the XT hub over the XTR just cause you wont really appreciate the weight savings so there's not much sense in spending the extra cash. Those rims were the rims used on the deemax for quite awhile but they now have a special rim that's unique to the deemax that has some extra milling along the rim's interior. You'll eventually want some hubs with a quicker engagment for DH but those will serve you very well for now.