tested today the combination of USARSim with UDK Feb 2012. Note that this was just testing a direct connection with a single robot (Nao), the WSS was not tested.
UDK benefits are shown in a 5 minutes video http://youtu.be/HpTSK9aBP1c.
We still use only a small part of the possibilities.
The advantages of the Feb 2012 release are given here:
The advantages mainly seem to be towards multi-platform developments (Mac, Mobile) and controlling the detail the world is visualized and doesn't seem to directly influence USARSim.