I'm light armored and I got the perk that increases my stamina regeneration by 50%. I haven't learned Stamina Regeneration yet but when I do my stamina will regenerate 90% faster.

I don't think so. I think the caps only apply to resistence.

No they can go higher. Some armors have 100+% regeneration rates. I have multiple 70% regeneration rate pieces amongst different pieces of armor. Some enchantments only can go on certain pieces...

If ur talking about potions too, I have been able to craft fortify smithing potions that improved weapons and armor 102% better. A 52% increase over the blacksmiths elixir.