Destruction was never an issue, even post level 40. All you have to do is have impact, incinerate, -100% destruction cost, and always dual cast. This is how I play my mage and nothing ever breaks through my line of spells to touch me.
Oh, and aspect of terror or fear, whatever it's called. Does wonders for fire magic.
The issue I have with it is that you need that dramatic reduction to mana cost to even make destruction magic viable/fun lategame, coming from someone who plays a level 59 mage and doesn't use magic cost reduction. I mean when I go up against an Ancient Dragon with 3071 health and my Incinerate does 157,5 damage and that's buffed with a potion and all the damage perks in the destruction tree it just makes me facepalm.
What I take isse with is the fact that apparently in the transition from Oblivion to Skyrim, mages now need great amounts of magic cost reduction gear to be able to even cast more than three or four high level or dual cast spells even though they pumped magicka on every single level up. That just seems wrong to me.