Once you get the 'Destruction spells cost 0 Magicka to cast' enchantments on your gear, you don't need to worry about taking the metamagic perks (e.g. That coupled with Conjured Storm Atronachs/Storm Thralls to keep your enemies occupied while you blast away at them has worked really well so far.
For my Breton pure mage it has been Shock Destruction all the way.
