Very early days, but I think the iPad Air 2's problems with sleeping on 5Ghz have been fixed with IOS9.1. Connection speed no longer seems to drop after a minute and it seems for now to stay connected. I have seen it switch to 2.4Ghz once, but not drop off the network as before. Anyone else confirm?
Jong, no iOS 9.1 hasn't fixed the issue. I did reset the Network Settings on the iPad Air 2, then connected to WiFi 5 Ghz, left it locked and been monitoring the connection from the router's webpage. As you can see from the pic the last one in the list is the iPad Air 2 - it keeps disconnecting and connecting back, and then Tx drops to 7 and then the device disconnects from the Network and connects back again after some time.