Sluggish performance with Airport gear on 5 GHz
The 5GHz is all hype.  It can be faster, only if you're close to it.  2.4GHz is still superior in range and as you know, WiFi speed drops off the further away you get from your access point. 

The only time I've found 5GHz useful is: When many other networks are interfering with mine. (in between two large apartment buildings with 12+ networks bouncing within the corridor, it's nice to switch off of the 2.4GHz)  

Or, if I'm in the same room as the AP, I can get a strong signal.  Then 5GHz is sometimes faster.
here's something interesting from wirecutter.com

No matter what they are called or technology they use, repeaters start out with a minimum 50% throughput loss. The reason is that a repeater must receive, then retransmit each packet using the same radio on the same channel and with the same SSID. If the repeater is very efficient, then your loss will be close to 50%. But if it’s not, throughput loss can be higher.”

Thanks to that 50% loss in bandwidth right off the top, just about all wireless extenders suck.  

http://thewirecutter.com/reviews/best-wi-fi-extender/