Gpyro has an internal cap on it based on 35% of the casters health. This is why you cannot 3 shot a world boss with it. It generally always does around 75-80k
Shifting it to pyroclasms position on the talent tree will not break anything, out side of just making it mainstream for the mages casting rotation, though, thats how it is already anyways.
In pvp, spamming it really hurts the fire spec, removing the identity of combustion etc. this is why i think it should go to instant cast charges. The ability doing large amounts of damage is ok, but it really should be reworked to being instant cast charge stacks, or some how interacting with the heat up mechanic.
It might actually be better to ditch heat up out right for clear casting, and shift all related talents etc to clear casting. In my opinion that is more iconic for the mage class, and will help fire potentially cast more often.
If greater was worked into that clear casting state, we’d see a much more fluid spec.