JMSIV

Bought used track wheels with R-compounds. Pulled the R's off and mounted Sumitomos (cheap). Tires have now been heat-cycled about 15 times, (3X track days). Tread thickness is still good, but the tires look like marshmallows that were left in the fire. Just driving them around, it seems that they have lost some grip, at least while cold. Are they toasted (literally?) or will they do OK for a few more track events? I am not quite ready to put on the R's yet.

Pete S4

Wouldn't really be surprised by what you are describing, track events, and heavy cars can do that to ultra high performance tires. It's fairly normal. Just keep an eye out for uneven wear, tread separation, and keep them properly inflated.

Don't know what kind of tires you are coming from for comparision, but generally tires on this end of the spectrum need to warm up a bit before they get maximum grip. Keep in mind I'm making general statements here, I don't have personal experience with Sumitomos.

MachtigAvant (v3.0)

Great for what they are, but they aren't going to take the abuse of 15+ heat cycles.