The problem is that ona 7D even the slowest card will be able to do the 8fps. You have a 15 frame buffer. The question is how many shots can you get off before the shooting slows down. By the time you hit 15 frames (almost 2 seconds), you could write like anywhere from 0 - 3 photos. Fast cards will write 3. But how is that an indication of much? 15 vs 18 photos before the buffer fills.
A better test would be how long it takes to write that entire buffer off. Or if you held the shutter down for 5 seconds, you could probably get 26-28 photos shot on a fast card. How long after that would you have to wait before the buffer is cleared?
Shrug. It all depends. If this thing performs at 533x or at least 400x speeds, it'll be pretty fast. No doubt 600x cards are faster, but there's a point where to most people, they will all be fast. So you really need synthetic benchmarks or real timed numbers where people tell you "I shot off 25 pics vs 28 in a 5 second shutter hold." That might be all that separates a 600x vs 400x card, but the average user won't be able to differentiate unless you benchmark it.