Glad to know that I'm not the only one who had problems with WRT1900AC's 5Ghz wi-fi under OSX.
I could only get 600Mbit/s link speed when either my 2013 Macbook air or iMac (both AC capable) is connected to the WRT1900AC's 5G wi-fi network, sometimes if I turn the wi-fi off then re-enable it again to connect to the WRT1900AC's 5Ghz network manually, it might show 1Gbit/s link speed for a little while before automatically changing back to 600Mbit/s, however during the period the link speed was 1Gbit/s, it was impossible to load any webpages (same phenomena on both machines).
I had really high hope for the WRT1900AC and I admit that its 2.4Ghz distance was indeed better than my R7000, but after tinkering and bearing with the WRT1900AC's disappointing 5Ghz wifi for 5 days, I gave up and returned it.
I will be keeping my Netgear R7000 which delivers true AC performance and has been rock solid with two AC capable Macs since day one (bought my R7000 about 4 weeks ago).