I agree with you on the magicka reducing enchantments. The destruction enchantmens should instead enhance the damage, like the respective potions do.
But as far as Impact is concerned, I really see no problem. If you don't like to stagger a Dragon, you should avoid to dual cast at it... archery can be incredibly powerful, destruction lacks immensly in terms of damage. That is why the Impact Perk is so valuable and have to be more potent than the respective marksman perk - imho.
I do share your opinion on Impact. I did say it is very crucial in many combat situations, and it should never be removed completely or so. But the real problem here is enchanting. It will turn that perk into something beyond godlike. If you wasted your magicka as you're ment to, you'd stagger dragon few times then run out of magicka. Like this, it's just really lame concept, and the reason why I would like Impact to be the same like Power Shot is only because I know it's easier to remake Impact so it doesn't affect dragons, instead of remaking the whole enchanting (Which, may I add, I would love to see, but that ain't happening).
And like I said before, just because the game is broken at some aspects, in this case Impact/Enchanting, I don't wanna adept to the game and quit dual casting, just because it's a mood killer. And +, I am having really hairy fights on master difficulty on this build, and I can't afford to lose very nice amount of burst damage just so I do not stagger a dragon. :/ It would feel like I want to lose!

But yeah, I can see where you're coming from, and I agree Impact is really handsome perk, and nearly a must for destro, but then again... No idea, it's a two way road, and however you take it it's a lose-lose situation in this, so I'd rather go for a lesser evil, in this case making Impact like Power shot (Since we will never force Bethesda to remake broken enchanting).