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...