I have a 2013 MacBook Air with 8 GB of RAM and it can edit 1080p HD video like a champ. Current generation MacBook Air should be able to handle full HD video editing just as well. Not sure if it can handle 4K video though.
There's video of the 2015 12" rMB editing 4K video..I'm sure the 11" with an i5 will be fine..
I had a 2011 i5/4gb/256gb MBA and upgraded to a 2013 i7/8gb/512gb MBA. Don't know anything about GoPro, but I edit Sony XDCAM-EX 1080/24p HD video and the 2011 machine handled that fine. The 2013 renders faster but otherwise is pretty similar.I imagine a 2015/16 air would probably fair better than yours though right?
Yes, I have a new 11" air and edit GoPro video without any problems at all. I haven't tried to edit 4k video yet, as I almost always stick with 1080p.