She's definitely level 27 but thats all I know off the top of my head. She's pretty much put most of her eggs into the destruction basket and I think that's where I'm going wrong tbh...
Because I've neglected restoration I didn't even know that perk existed

I need to look at this a bit more broadly I think...
Or look at enchant. My Destruction mage did not take any cost reduction perks but put those into enchant instead. Right now, he is level 32 with some Destruction robes that reduce destruction cost by 12% and increase magicka regen. Plus he has enchanted a ring, amulet and circlet with Destruction cost reduction at about 23% each. He also has a ring that reduced Restoration cost by 23% so he swaps that with the Destruction ring. He just picked up the Ward Absorb perk in Restoration, so now he can hold up his lesser ward against mages no problem when wearing the Restoration ring and blast away with firebolts and flame spells for a long time.
The basic flames spell can be really powerful if you take the augmented flames perks and take the Aspect of Terror perk in the Illusion tree. I can now do 27 points of damage per second for almost no magicka with Flames, plus if you shoot it in bursts, the burning effect stacks and you can do quite a bit of damage in short order. When I am in close quarters, I use the novice spells more often than any other spell, saving the firebolts for when I am at range. If I am having trouble hitting someone or if they are in groups, I switch to an AOE spell but you have to be careful because those use more magicka.
With the cost reduction through enchant, I am having no problems with a Destruction/Restoration mage. When I get Enchant to 100, I will be able to put two enchantments on a single item. With that you can get free castign in two schools, but that seems cheap to me, so I will probably just go for substantial cost reduction. You could get a 75% reduction in Destruction and Restoration, for instance and a 25% reduction in Illusion and Conjuration, or you can get 50% reduction in four schools. Since you can wear a circlet and a falmer helm at the same time, you can reduce spell costs even further. You can also combine in game items, like the Arch mage robes and the savant circlet and you can fortify your enchantments up to 29% with alchemy to achieve a wide variety of possible cost reduction scenerios through enchanting that will save you a great many cost reducing perks.
Plus, with all the perks I saved by not taking any cost reducing perks, I have plenty to perks to perk out Illusion (9 perks) and Conjuration (4 perks to twin souls) as well as Destruction and Restoration. I am not bothering with the magic regen perk in Restoration, since a mere 50% increase in regen seems awfully expensive use to two perks to me.
Likewise, I am not perking Alteration. My mage does not wear armor, but constantly castign a mage armor spell seems tedious, especially since it only gives limited protection even with the three perks in mage armor. I'd rather just use Wards and focus on not getting hit, than worrying about mage armor. Personal preference there. I may spend two perks there to pick up dual casting someday, just to make my candlelight last longer. Its annoying to have to recast that all the time.