I did a google search for this but came up with nothing.

All of the restoration spells dealing with undead have a level cap shown on them, with the highest being the masters at 44 and 35. I know that with the dual casting perk it doubles the level, so that is 88 and 70. With the 81 cap, what use is a restoration spell that will no longer help because undead are now 81? Or do restoration spells level with you? I am asking because I don't want to invest a lot into the restoration tree if the non healing spells are going to be useless at max level.

To put it more simply, do the repel/turn undead spells become obsolete as you level up?

Enemies stops scaling at lvl 50 (in the base game, dunno about Dawnguard yet), so you'll still be able to hit undeads with those spells when dualcasting or with Necromage.

I had no idea they stopped scaling. I was under the impression that certain monsters, like Draugr Deathlords, continued to scale up to max. Thank you for that information.