If you stack up -75% manacost you can pretty much cast dual-casted destruction spells infinitely.
Taken this into consideration is destruction really that bad at higher levels?

Also.

Does the +25% fire damage perk in destruction add to the damage of flame atronochs and so on?

On my old mage characters I hate duel-casting cause its boring as hell. I'll mix it up with a sparks spell in the other hand or an ice storm spell or something else. I think the main problem with destruction is they should allow it to the same damage on master as it does on adept, because destruction is actually strong on adept difficulty, I don't think most would disagree with that, but in order to do that they have to nerf destruction-enchantments to do same damage they do now and nerf potions of fortify destruction.

To many 100% reduction means 'overpowered' so that's not always an option for them as they want a challenge; which typically requires playing on master. With the damage master dmg reduction its obvious why they're upset... Runes do 25 damage. Thunder bolt does 45... I can feel their pain too. Imagine, dual casting the costly thunderbolt at a regular bandit Orc on master only to see him take a knee. Very annoying indeed.

However adding more attack damage kinda ruins the natural balance of mages on both sides. People playing a pure warrior would have to go Breton, loadstone, and resist magic necklace in that order or invest in the block tree if they wanted a mere chance at even getting close for a swing at any Arch-Mage. Otherwise, on master, they'd get blasted in one to two hits. If your a vampire especially on PS3 then Novice fire mages would now pose a notable threat. On the flipside Players playing on adept and advancing normally for a moderate challenge would become somewhat too powerful.

So it comes down to what kinda scaling people are asking for? 5 pts per 3 levels? It'd be easier if destruction was simply unchanged through out the difficulties.