A cool shirt helped a ton for me when I was racing, but I've seen air temps of 109 deg F at the start of the race before!
I try to do a little mental mantra on each straight as it prevents me from drifting off into la-la land: check your mirrors (coming into last turn before straight), check your gauges, breathe, relax.
I've ran a 2-hour "enduro" at Phoenix International Raceway (1.5 mile roval circuit) in cold weather (no cool shirt) and it was no big deal. I've also run 1 hour at LV Speedway (old roval circa 2000 or so) at the end of a 3 hour night enduro where the air temp at midnight (checkered flag) was 100 deg F -- no cool shirt -- I was wiped -- I'd also run a practice, qual, and sprint race earlier that day in the heat. With a cool shirt I'd have been good to go.
'11 E90 M3, Mebourne/Black/Fox Red/Black, 6MT, roof, PP2, heated seats, moonroof, split folding rear seats, iPod/USB adapter -- 8/8/11 euro delivery! Technic harness, JL XD 6/600.