I've updated my ASUS RT-AC66U Merlin's asus-wrt 372 firmware. I'm running at 300 Mb/s in AC mode (80 MHz only). I still thinks it's the antenna, though.
FYI: someone else mentioned having the RT-AC66R. It's the exact same router. The R simply indicates that it's from Best Buy. The firmware I'm using will run on that just as well as on the U version.