You know, I love min-maxing. But since I'm melee I'm no super expert on magic. However, I did try it a bit with left-over perks for a bit (without saving) and what I realized and what I never read about it: why aren't you pure magic users investing heavily into alchemy? You can make fortify destruction potions to increase strength by a lot. You can even favorite and hotkey it. There's no excuse (aside from running out of ingredients to create them). Dual casting adept/expert skills + perks + potions = pretty darn powerful.
Also, low level magic users should invest heavily in staffs. They are meant to be used when you run out of magicka: they get all the benefits from perks/skill level/fortification. Not sure if you can dual cast though, probably not.
If you want to play a pure Destruction mage without using enchanting to get free casts (which is equally gamebreaking by allowing an infinite stunlock against a single enemy, even if the damage is weak), then yes you will need alchemy. But that'll just bring it up to the kind of damage you can put out normally with other game styles. And remember there are potions to increase melee/bow damage too.
Theoretically if you're willing to use the fortify restoration/enchanting loop you could make an arbitrarily powerful fortify destruction potion that balances out the damage. It's still a broken balancing issue by forcing a player into certain perks to compete with what others get for nothing.
Damn, so what does this mean for a high-level mage? My Dest and Conj are leveling pretty evenly, but obviously I do the other magic types like Alt and Rest as well. Do I have to use bound weapons at some point?
Conjuration seems to scale quite well, though at high levels you end up with a few problem with raising dead, as certain enemies will end up too high a level to raise (a pretty stupid restriction but easily fixed by mods if you have the PC version). Bound weapons start off very well but are less effective at higher levels because you can't enchant them or improve them with smithing.
Alteration: armour spells are quite ineffective at higher levels, save for the Master one which is very effective but impractical to use. Paralyse is extremely powerful though, and borderline gamebreaking in my opinion.
I haven't found any huge problems with Restoration, at least with healing spells. If I remember rightly though the turn undead spells should become useless at higher levels because they only work up to level 30 or something, but don't quote me on that.
Illusion: all over the place. Until you unlock the perk that allows it to work on almost all enemies it's kind of ineffective a lot of the time, but powerful when you can use it. With the perks you can get it scaling pretty well until max level, though you might end up with a few enemies that are too powerful to affect. With perks and dual-casting though, pretty much anyone can be influenced.
The other schools do suffer from scaling problems but they're no way near as apparent, and generally can work very well up to max level. Destruction is the main problem.