To begin: I'm 6'0" and right at 200 pounds and wear a L-XL II in this Assos shirt. I also ride in South Florida, so the conditions are pretty extreme.
I wear the long sleeve version, and for me I feel noticeably cooler when I wear it. When I open my jersey a little, it makes an even bigger difference (I've since this purchase bought an Assos Millie jersey that I also love).
The sleeves are a little long, but because they're so tight you don't notice it. This is probably a function of me being a larger rider relative to my height.
Cool shirts and undershirts are a tricky thing. Your mileage may vary, and you might not like the idea of wearing the shirt under your jersey. For me, riding in 80-90 degree temps every day, I love it. I've actually bought a second one so I don't have to wash them every day and I can rotate them.
Familiarity: I returned this product before using it
I had the previous generation long sleeve skin foil base layer for Spring/Fall and it fit like a glove in a size I. Went ahead and ordered the Summer long sleeve version in the new generation skin foil in exact same size and the fit is totally different/off/bizarre. Ok in the abdomen and arms - in fact the sleeves maybe even a tad short and definitely snug (moreso than the previous generation). But the upper shoulders and neck feel 2 sizes too large and sort of billow there while the lower half fits snuggly. I get that Assos tailors garments to fit in riding position as opposed to standing in front of a mirror and the billowing does chill out a little in a riding position but not enough to make any sense. I am not a rail thin dude and for a cyclist am pretty muscular in the shoulders and chest relative to my waist size so this fit makes zero sense. May exchange for the size 0 but would be worried the sleeves will be waaaay too short and tight then. We will see. Scratching my head on this one Assos...