I just switched my R310 from solo to unleashed to see what the configuration would look like. On the firmware I was using, the 2.4 and 5ghz were separate SSIDs - I didn't see a way of making a single one that was both. I know on some manufacturer APs it just switches depending on the signal. When I set it for Unleashed, I didn't see a separation any more. Is it both 2.4 and 5ghz on the same SSID?
It’s a client side decision, but yes your devices should connect to 5GHz if the signal is strong and interference is minimal. Apple devices for example will favor 5GHz and on macOS always defaults to 5GHz as long as the RSSI is -68dBm or better.