I have it laying on my table, did not have a chance to install it so far...
you can get the antenna diagram from here:
http://i.mt.lv/routerboard/files/SXTac-140718123939.pdf
Anyway I have experience from normal SXT AC running as AP doing PtP link.. was not happy about it, when I've replaced it with NetMetal5 connected to dualchain panel antenna and left the client side on SXT, its running great so far.
The previous SXT N based should handle 30 clients with reliable speed. My NV2 AP (RB711) mounted on UB** omni runs great with max throughput around 60-70Mbps TCP, currently handling about 16 clients, its almost same radio s the SXT so it should be working fine, definitely an improvement from CM9.