Did a few test on dummies.
Thunder III
779
803
817
806
1173 (crit)
761
Thunder I
758
802
764
764
786
Not enough cast to mathematically prove anything, but it's enough to convince me that Thunder III Thunderstorm does indeed do more damage than Thunder I.
Irrelevant anyway due to the fact that Thunder I is the ideal filler for regen phase.
