Actually, I've had the very opposite experience with the 215's. I, along with most people I know that have run these tires, feel the 615's handle the heat better. They will overheat, as you stated, but I feel the 615's are more forgiving regarding heat. The 215's have a much stiffer sidewall, and are heavier because of it, but they have a better turn-in response. I have also run a shaved Kuhmo MX and the 615's are a better tire. You also might check out the Bridgestone RE-01R, supposedly a very sticky street tire. Just became available in a size that I can use on my autocross car and I'm going to try it this weekend.
My experience with these are on a STS Civic for the past 3 years. I'm now just starting to build an XP 240z.