With 100 enchanting, +enchanting potions, and crazy +enchanting items, you can get up to 100% off on destro robes, which means you can infinitely cast master level spells with no mana loss. It's not even gamebreaking on Master.
You sure? In the video I saw of a guy doing that, he was still losing magicka, just really slowly. I think it's probably capped at -85% like most things.
