Need More Power - giving fireball more boom

[Drakenbow]

I have been looking at how TFT spells shift from having a one hex effect to a 7-hex effect cost-wise for the caster's ST.

• *Fire 1 > 4
  *Illusion 2 > 5
  *Wall 2 > 6
  *Sleep 3 > 8
  *Avert 2+1 > 3+1
  *Image 1 > 4
  *Shadow 1 > 3
  *Freeze 4 > 12

The average upgrade is about 3 times the base cost to make a 1-hex into a 7-hex. Now applying that to a Fireball, it seems like a nice upgrade to allow for a caster to power-up from hitting one hex to hitting a megahex but spending 3 ST per die rolled (maximum of 3 dice rolled).

Has anyone tried this as a House Rule or something like it?