This is good to knowI went ahead and bought a class 6 card and I could tell no difference between my stock 16gb card and my class 6 card. I bought an 8gb class 6 and ended up going back to my stock card because I could tell no difference. I am also running it with JIT which I was told should not work but it does on the Koush's stock ROM. .I've been running it on the stock sd card for a while now (well since he released it on ROM manager). Going into it I didn't really care to potentially ruin a card, and I have everything backed up, but thus far it hasn't even remotely been a problem. I haven't tested with class 6, so I can't give a comparison, but it's running perfect for me.
The way I tested it was by loading RaceThunder2(20MB) and it ran the exact same on both cards, or should I say loaded the same, I also ran several other programs and there was no difference that I could tell.