When moving up the step height, you may want to just try doing a couple of the intervals at 6-inches. Imax 2 is easy to do this way, since the first 4 intervals are somewhat easier (intensity wise). Maybe each week try doing, just one more interval at the next height.
ETA: Be very cautious on interval blast #6 (straddle tap repeaters) when you up the step height!!!!! This one is tricky when you are accustomed to a lower step!
Imax 3 is a completely different creature, because the choreography is more complex and Cathe moves so quickly. I would definitely take Imax 3 in baby steps and learn it on a lower step and then work your way up to a higher step.
FYI - I do Imax and Imax 3 on a 6 inch step and Imax 2 on an 8 inch step. Most of us will tell you that Imax 2 is a little easier than the other two.
Good Luck,
Tina