EX8000 - is there a way to configure the extended 5Ghz channel (from default channel 44)?

You're caught in a catch-22. The EX8000 uses 2x 5ghz chips. One is higher 5ghz and the other is lower 5ghz. One is used for backhaul connection between router----extender and the other is for extender----devices. 

And because of channel overlap, you're going to be overlapping those channels regardless. (i'll attach a diagram showing channel overlap)

so either the backhaul will be overlapping on channel 44 or the fronthaul will be. One will regardless. 

On the plus side, 5ghz doesn't broadcast very far or through much materials. And its much much faster than 2.4ghz. 

So it suffers from much less interference issues than 2.4ghz. Even though there's other networks that use that channel, it won't take the samer performance hit the 2.4ghz channels do.