Thinking more on how to balance destruction magic in the end game, I've found two major complaints: Damage is too low, impact is too good.

What I propose is that the impact perk also give you a chance to critical your spells, but also reduce the staggers to the same chance. Perhaps make the critical damage x1.5 (no more than x2) and have the chance be skill/2 so at 100 skill you have a 50% chance to stagger/crit. This way you could also tell if your spell crit because the enemy would stagger and you wouldn't need a notification like stealthers.

What do you think?