I'm planning to do a full playthrough as a nuker mage, and while I've been investigating different spell combinations, I've found discussions, from over 2 years ago, about fire cancelling the effects of ice, and vice versa.
Was this something that was originally in the game and has been removed, or did I just not notice the last time I used Primal spells? I started playing DA:O a few months after DA2 was released, so I never got to experience "other" versions.
I was reading one discussion about mage builds were someone essentially said that fire cancels the effects of ice. Would this mean that if I used Cone of Cold to freeze an enemy, then used the fire cone thingy on them, the ice would melt and the effect of Cone of Cold would thus last for less time? I'm sure I did that alot during the only time I used a nuker, and I didn't notice it, though I do remember wondering at the time. --- "You can lead a man to knowledge, but you can't make him think!"
if you're going to be a nuker mage, then there is no reason to pick up the ice tree. go fire + lightning and have the 2nd mage in your party pick up the ice tree for frost weapons and control when you need it. this also allows you to do the storm of the century combo a lot easier if you're into that sort of thing.
as for your question I tried using fire blast on a group of enemies that were frozen by cone of cold and there was no damage difference between them and non-frozen enemies. the frozen enemies did thaw immediately, however. if i were you then i'd just pick up glyph of paralysis/repulsion and use those to cc any troublesome enemy packs. lasts longer than the freeze affect, doesn't break on damage, and the paralysis combo is instant so your melee can rush in right after it's been used.
Off topic but cone of cold is one of the funniest spell I think, one of the very few where you want to position your mage instead of just casting spells from a distance. And I like those moments : - Positioning, positioning, cone of cold -> yay, I've frozen three enemies :D - Careful positioning, cone of cold -> crap, I've frozen Alistair, very sorry about that :'(