My understanding is that the BCM43364, and its predecessor the BCM43362, do not support MIMO.
Unfortunately, the BCM43364 does not support an RMII/MII interface either.
That's correct. The BCM43362 and BCM43364 are 1x1 11n wlan radios operating in the 2.4GHz band.
RMII/MII will be available in the BCM43907 chip that will be released shortly.