First I found out the bug/glitch/exploit in the damage over time spells where you can cast unlimited damage for absolutely no mana cost, now I stumbled upon this weird phenomenon.

Using dual cast perk for more "powerful" version of the spell only seems to drain my mana faster. I am left with less mana doing less damage than without the perk.

http://www.youtube.com/watch?v=33oZ8fmRci8

Wouldn't be surprised... I've tried the Dual Cast perk, and not really noticed any noticeable damage increase.

That being said the games so simplified you have no way of knowing your damage, stats etc...

So I literally don't even pay much attention to the perks, as I've yet to see one actually show me any actual increase in damage off what the naked eye can see.