I wouldn't say they make you feel cooler, they make you feel fresher as you are not dripping. I prefer them paired with Pro Team jerseys but they work just as well with sports wool. I used to swear by the merino base layers until I gave these a go, now the merino is for winter only.
Do they genuinely make you cooler than not wearing one in the hot then?
would be wearing under a sportwool jersey most of the time