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.