First keep in mind that all the equipment is half-duplex, and can only send to one client at a
time. this means its impossible to increase capacity by trying to seperate CPE’s by Antenna chain, E.G. 10 clients chain 1 10 clients chain 2.. the AP will still only be able to max out at 150 megabits total. it cant work that way 
If you run dual chain at the AP, if you run single chain at the CPE, you will only get
150 Mbits max air rate on that radio in any configuration… you would need all clients to have dual chain antennas to get a increase in overall capacity at the ap. The advantage too though is you would see higher signaling rates on all the CPE’s but if you had 1 single chain cpe connected.. the more that user is communicating, the less the overall ap capacity is.
Example:
User: Tx/Rx RateUp/RateDown CCQ Tx/Rx # of Chains
Client 1 -64/-62 235Mbit/270Mbit 95/92 .. 2
Client 2 -69/-71 208Mbit/235Mbit 87/82 .. 2
Client 3 -75/-75 81Mbit/81Mbit 82/79 .. 1
Client 4 -75/-75 162Mbit/162Mbit 79/77 .. 2
You can scale this up to… say 30 users if you want.. so say you have 29 dual can CPE’s with decent signal, and then one or two single chain (like Client 3)
When client 3 is downloading a 600 MB Linux distro at say 5 megabits, The AP has to spend twice as much airtime communicating that data, as it would with a dual chain client with similar signal strength (Client 4).
So really, all CPE need to be dual chain and good signal to gain most benefit from dual chain, so you can maximize the speed at which clients finish their communication. and will raise your overal max capacity for that specific AP…
So if you are trying to do a roll out of dual chain by starting with the AP, every user that you switch over will increase your overall bandwidth, and you will eventually gain your full capacity by switching your last client out with a dual polarity CPE. This would either allow you to supply more bandwidth per subscriber to keep up with increasing demands, or connect more users to an AP, (especially W/Nstreme 2)