This means that your links will use tcp extensively. tcp protocol requires a lot of CPU resources. pick your AP router based on the number of customers that you will have and based on the desired bandwidth that you want to offer.
If you have a handfull of customers then each RB with MIMO card will suit you. The more bandwith you need, the more expensive it gets and a RB435G may become your best choice - it has a fast cpu to handle the traffic and gigabit port.
If customers are in one direction use MIMO panels/sectors. If they are in multiple directions use two wireless cards in your RB but choose different frequencies for each link. If they are in all directions I recommend looking for a MIMO omni antenna - quite rare but already available.
Here is some further reading for you:
http://www.conniq.com/WiMAX/mimo-01.htm
http://forum.mikrotik.com/t/best-mimo-setup-with-nv2-and-tdma/50561/1