Yes evaporation does cool you down but it only occurs if the moisture comes into contact with air in order to evaporate. So unless you're riding with no shirt on, you must have to have a way of pulling the moisture off your skin to where it can evaporate. Here's an extreme example...say a person is wearing a rubber shirt as apposed to a person wearing a moisture transfer fabric. Which person is going to have a better chance at sweat evaporation. I do agree with you that it is more important to wick when it's cold but, That wicking is also to promote evaporation. In the cold weather case, the evaporation is important to keep you dry not cool. Being wet when cold leads to chill. wicking clothing does not descriminate when it's warm or cold. It wick to allow evaporation whether it be to cool or to dry. (Just my two cents as both a cyclist in the summer and a skier in the winter I've experience both extremes.)
Performance fabrics are not always compression. I have a couple of wicking loose t-shirts for hiking as well as a couple pair of wicking boxers. Base layers aren't always compression either. It actually depends on the use. For activities that are more...well...active, a skin tight layer is more useful because it provides more skin contact to maximize moisture transfer. It also gives a little muscle support(not sure if it will increase performance though). for other activities, the clothing can be semi-fitted to loose. For backpacking/hiking, base layers can be loose for comfort(subjective) and can double as outerwear in camp like for PJ's or just a regular shirt. And also like I mentioned earlier, there are regular clothing pieces that are performance fabrics.The Taget copy that I have is part of the Champion C3 line. Most performance fabrics are polyester or some polyeser lycra blend. Performance fabrics come in all kinds of names but they all usually boil down to some polyester content. Base layers are usually performance fabrics and are worn under outerwear, but there is outerwear that are made of performance fabrics. In actuallity, there isn't a difference. Base layers are called such because they are worn under something. You can take a wicking t-shirt and wear it under a jacket and as long as it's the first layer next to your skin, you can call it a base layer. The destinction between what is a base layer and what is not creates too many headaches. It's all performance wicking clothing to me. As long as you use it for it's intended use it should be fine. Your underarmour compression top may work just fine in the summer if it's not too thick. The only difference, I believe, between the underarmour heatgear and coldgear line is the length of the sleeves as odd as that sounds. I hope I didn't confuse you further.