I'm not sure I can cite a specific elevation, but I feel like anything more than a 1.5 club adjustment starts to feel a bit forced. 2+ club downhill holes seem to be more about getting the golfer to say "I played a really cool, elevated par 3 the other day." I guess it is okay if the hole is needed to transition between two distinct elevations on the land, but it still feels gimmicky to me.
Two examples for me:
Devil's Cauldron at Banff. It was probably a club to a club-and-a-half for me. It fit the land very well, and it is fun to play. Doesn't feel like too much.
Sugarloaf in Maine. There's a par 3 on the back nine (I think it is #11 or #12 but I don't recall exactly) that feels like you are standing directly over the hole and just need to drop something off the shelf. If I remember correctly, you also had to climb a good bit to get to the tee just to then proceed back down to the green, so it didn't feel like that decision HAD to be made in the routing.
I'm not sure how much elevation differential there is between these two holes, but I'd say somewhere in that range defines the difference for me.