Hi Katie,
I used to do this one with the 8"; I used to do all her step workouts at that height. When my left knee began to talk to me, I lowered my step to 6" for the workouts that Cathe does on 6". When I lowered the step height, the workout felt easier, and I think I improved my form quite a bit. Interval 6 can be very tricky on the 8"! If you're short in the leg, like me, there's no room for sloppy moves.
However, in terms of intensity, the most intense moves in that workout are done on the floor; step height is irrelevant. Where you'll notice the difference is in the aerobic (the easy step part prior to the anaerobic intensity blast) portion of each interval. If you find that you recover quickly from each intensity blast, and that the following step aerobic segment feels easy, then using the 8" step would up the intensity of the workout and create a new fitness challenge for you. If, however, you are still using the step aerobic sections as extended recovery periods, then you might want to hold off on increasing the step height. This is the long answer.
The short answer is that you have absolutely nothing to lose by trying the workout on 8". No one says you have to stay at 8" all the way through; give it a whirl and drop it down again at any time that it's feeling too intense. You may pleasantly surprise yourself!
Let us know if you try it!
Sandra