Here is some logic for you.Originally Posted by Zærkalla
Spellpower directly increases the damage of the spell.
The value of haste is increased by the damage done by your spell.
Haste has a cost. That is, to stack more haste, you would need to not stack spellpower.
Therefore more haste = less spellpower. (nothing is free right?)
Less spellpower = less damage done per spell
Less damage done per spell = less value on haste.
I'm not sure if i'm saying this right... but..
Since the benefit of stacking haste comes at a cost spellpower, and the buff is a %based damage increase, The overall increase or decrease of damage would be the SAME as if the buff was never there.