My Extreme is the latest generation model, and the slingbox 500 is configured for the 5GHz band. The Airport Utility 6.1 reports 802.11a/n, no different than many other devices on my network. My qualm is that the signal strength of the 500 is particularly weak in comparison to other devices in the same room, but it still is able to stream 6000-8000 to the browser client. and 4000-6000 to the iPad. My other qualm is that the slingbox won't connect when the WiFi is configured as hidden, so I have to broadcast it just for the slingbox to connect.
I see from your image that your rate of 54 is consistent with the max rate that 802.11a is capable of. In my case, I see a reported rate of 120 Mb/s, so I believe it is actually connecting using n and not a. If I were troubleshooting it, I would power down both router and slingbox and try relocating the router close to the slingbox, then power up, and check to see if it initiates a connection with n.