I have Cool Wings and have used them twice on hot afternoon rides. I had the same issue as OP with a little gap in the front between the jacket and the sleeveless jersey, but it didn't make any difference when I was riding.
On Sunday, it was mega-hot here and I wore them for three hours. I poured water on them a few times
. They dry super fast. I didn't get sunburned like I usually do and like the OP, my arms stayed dry. My legs on the other hand were covered in sweat and salt and despite sunblock I got a little too much sun.
I've worn them with a racer back top as well as a sleeveless jersey -worked equally well with both.
Overall, I am going to wear them all the time if it's hot. I think they are worth the investment.
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"I never made "Who's Who"- but sure as hell I made "What's That??..."