If anyone is willing to test and helps you in finding the problem. If can do a real life 5g wifi speed test, not just iperf. I' ve compiled two images for the wrt1900ac off the trunk today(july 7th 2016).
The builds are identical except that one uses mwlwifi drive from March and the other uses mwlwifi drive from June. The builds have ssh and luci gui
https://drive.google.com/open?id=1S1QyTzrPid53y18u7f7AWNCQ-jKWHNvQ