Mental or physical..
Theoretically (Maybe hypothetically idk..) the best Jon ever will sound singing BOR, Lost Highway, etc:
He will sound like what he currently does if he doesn't change anything (Using twang or whatever you might do) and might be a bit flat sometimes (Depends on how high the note he's hitting is of course.) but he won't crack, be inconsistent, be out of tempo or anything like that at all.
I think that's easily listenable if it was like that.

If he sounded like 2011 it would have been good enough for me.