Hi Tom, tread remaining is almost valueless in determining the health of a bias race slick. The number of cycles you get will depend a lot on car weight (not too much in this case), and how you drive. Did you wail on them from the second corner, or build up to temperature over a few laps? Did they cool completely before you wailed on them again?
If abused, they'll be (relatively) slow after as few as three cycles. FFord and F2000 guys are on Hoosier radial slicks and getting 6 cycles without a significant drop off. Of course, each guys measure of significant depends on where in the pack he expects to be LOL.
And just to note, and I'm sure everyone here knows this, one heat cycle per season does not mean your tires are good for three years :-)