Do you have a long cage derailleur with a 50 or higher tooth count?

They pretty much all look like this. If the top pulley isn't rubbing, it's about right.

If he added a link or 3, it'd droop in the smallest cog.

Personally, I'd be perfectly fine with an 11 or 12 tooth smallest cog. Even for fast downhill. Seems to me the 10 tooth is ornamental except on the occasion downhill paved stretch, which these bikes don't do too often. At least it gives me a better chainline in the 12 to 17 area.