If I'm at level 81 what are the level's of enemies I'm going to face. I think I heard they cap at level 50 and all the spells I look at like turning undead spells don't go higher than 30 or 35.

Does this mean that since I'm at max level these spells will be ineffective against everything?

Well enemies tend to scale every so often but I believe the scaling stops at level 50 so it doesn't matter if you are 50 or 81, although with the Dawnguard expansion, they did add new dragons for 50+ so I'd watch your back.

Your spells should be fine.

Lvl 30 spell + dual cast = lvl 60+ spell....you're good except maybe against some new dawnguard enemies....not sure what they scale to

So this master spell is pretty much worthless on my max character?

Bane of the Undead
Spell ID: 0008c1ab
Tome ID: 000dd647 100
Master 1200 988 Sets undead up to level 30 on fire and makes them flee for 30 seconds:

• http://www.uesp.net/w/index.php?title=Skyrim:Stagger&action=edit&redlink=1, 0.05 pts in 80 ft
  
• http://www.uesp.net/wiki/Skyrim:Turn_Undead, 30 pts for 30 secs in 80 ft
  
• http://www.uesp.net/wiki/Skyrim:Bane_of_the_Undead, 50 pts in 80 ft

No, because Deathlords are level 30.