iirc it can actually exceed that; the damage is standard based on the level, and a higher level increases the likelihood of being hit back.thorns is absolutely broken it basically does half of the damage you deal back to you
Personally, while the enchanted golden apples are massively powerful, I'd say that the sheer cost balanced out the huge buffs it offers.Go ahead and remove notch apples and poison potions, possibly the strength potions as well.
I found this plugin which can set limits for individual players and separate enchantment levels, and also apply caps to individual items. It also scales the 30-level range to match the new maximum. Even if nothing remotely similar is used, it's probably a neat sort of mechanic to have in some way shape or form.