A number of the Qualcomm-Atheros Wi-Fi chips include support for external antenna switching which allows one to connect a different set of antennas to the chip's antenna terminals based on the station with which it's communicating. Vivato used this capability in the early 2000s. Ruckus has used this capability throughout their product history. More recently, Google's On-Hub uses this capability. In each case the result is several dB of antenna gain over MIMO alone.
Last year, Vasanthakumar Thiagarajan (of Qualcomm-Atheros) uploaded a patch to the Ath10K driver to provide support for smart antenna switching, but I still need to know if there is any access to the GPIO pins where the switch