diffrent strokes for diffrent folks... Am i talking to an one way mind???
The OP wanted to know if the flames spell was viable later in the game.
If that is the only spell he wants to use, impact doesn't work, and equipping the spell to two hands without the dual casting perk is more efficient than if you take the perk, in which case you dual cast at +40% magicka cost, to get only +10% damage. If you take firebolt, which the OP didn't mention,
then taking dual casting to get impact is worth it, as the stun lock can then be applied.