During the Everest Challenge, I always wear a full zip jersey so that it can be fully unzipped for maximum air cooling effect across the front and sides of my body, especially at lower elevations where it can be quite hot.
This year, I was baffled to see so few riders using a full zip jersey this year (zero actually, though I was told some were), because my own experience leaves no doubt that fully unzipping results in significantly increased air cooling, which aids performance.
My sense during a climb is that power output will drop from ~290 watts to ~240 watts when the temperature rises from ~60°F to 90°F. That’s a huge difference!
Just today, I came across this fascinating article on the subject of keeping body temperature lower for reducing fatigue and keeping performance high, which dovetails neatly with my own experience:
Definitely worth a read.