The fastest I've ever been on a course was around 50-55mph because they had a semi-long straight with a corner that I didn't have to lift on into the longest straight of the course.
When Jason and I were running the last event for OKC, we were debating as to stay in first for the beginning part of the course or go into second. Since he had his runs earlier in the day before I did, I got to watch him go through the course and hear the shifting. Needless to say, he was on average I believe
half a second faster when he stayed in second.
At the last autocross I went to, you needed to go into first gear since the track was as follows:
There was a downhill slalom into a 180º hairpin (at the bottom of the paper) that brought you back uphill through the aforementioned slalom. Averaging only 10mph or so around the hairpin cone and then going uphill, it would be detrimental to your time not to go into first. You could see this most obviously with the automatic cars since they didn't want to go into first gear, and you would see people putting up the hill in second.
From what I've read, seen, and experienced, most of the time going down into first will cost you more time than it will save you. If you think about how long you're at wide-open throttle versus the time you could be spending braking, turning, and accelerating in second, you've gained the time you've lost in the slightly slower acceleration. I've had this debate in an ST vs. SVT thread about whether it would be faster to downshift into first or to stay in second gear when traveling around 30mph. Why would it be any different from an autocross than on the road?
I need to start left-foot braking.