We're primarily concerned with the potency changes that come from with Astral Fire III, which is a 1.8x multiplier, so let's call Fire
a 324 potency spell. Fire IV is 504. Thunder III slowcast is 70+240 = 310, but has a 47%* chance to give you an instant cast 70+240+240 = 550; further, you can force the proc with Sharpcast. I don't have the Thunder II/IV maths off the top of my head but the idea is the same, just with a smaller chance but on more targets.
* 10% chance to proc on first tick
plus 90% chance it didn't proc on first tick × 10% chance to proc on second tick
plus 90% chance it didn't proc on first tick × 90% chance it didn't proc on second tick × 10% chance to proc on third tick
plus ... etc