There is new chip from Atheros AR9380 with 3x3 MIMO. New cards are coming in December this year.
Normis, are You planning to make such board? 11n support with Nstreme v2 is nearing with final ROS v5 so...

Don't know if AR7161 used as CPU will be suitable for such large amount of traffic from 3x3 MIMO. So maybe new CPU is needed.

We have tested 3x3 MIMO and the difference is too small from 2x2.

Regarding the subject, RB800 does have a miniPCI-e slot.