You didn't give us really much information about your test setup.
A quick test thrown together last week gave something about 23 to 27 MBit/s half-duplex using NStreme and WPA2 encryption between two routerboards. But as this was with bandwidth-test server and client running ON the respective routerboards (which alone eats significant CPU resources), higher throughput should be possible.
I think figures slightly above 30 MBit/s half-duplex should be possible using RB500s.
But tell us more about what you config looks like in this test - perhaps something else is using too much CPU...
If you really want to squeeze everything out of a point-to-point link using NStreme, be prepared to get something with more CPU power (like lets say 1 GHZ or more) - NStreme IS quite CPU intensive.