It is my understanding that the MWDA uses Miracast which uses Wi-Fi DIrect to establish a peer-to-peer Wi-Fi connection and thus doesn't go through a router. If this is the case, then why is the GHz of my router even a question? In any case, I'm using the latest Apple AirPort Extreme, which I assume is operating at 5GHz.
So, yes it is direct. But, if you are connected to the router at 5GHz then the connection to the MWDA will be 5GHz. If you are connected to the router at 2.4GHz, then the Miracast connection is 2.4GHz.
Apple by default uses the same SSID name for both bands, so unless you changed manually. You could, as a test, force a 5GHz connection .. or 2.4. http://answers.microsoft.com/en-us/surface/wiki/surfpro3-surfnetwork/specify-24-or-5-ghz-wifi-bands-selectively-on/0015e765-92ce-40c8-923b-9347b398f1b0 .. could you let me know results both ways?