It seems to me that if anything as you become more adept (no Skyrim pun intended) at destruction, the higher the spell the more damage per mana. The damage fper mana actually goes DOWN as you learn higher level spells. Heres the problem with how Skyrim built Destruction magic, particularly on high levels of play like Legendary which is what I play on. I added the mod 'Improved Destruction Magic' To me it doesn't go far enough, but it's still a definite improvement in that it does increase the damage per mana of a lot of the spells.
