The real damage shift comes with Rain of Fire / Fire Storm -- changing 3 to 4 or to 5 is a real nightmare of difference, particularly in the same element. I think the card should limit the scope of damage to targettable spells.
Also, the upgraded card should either have the same damage impact or be more expensive. A +2 boost makes Fire Bolt hit for 5 damage instead of 3 (without boosts). It would make Thunderbolt hit for 7.
