Isn't much of the UI rendering in OSX now dependant on the GPU? All the transitions, transparencies, scaling etc. I know the UI on the latest MBA feels a lot snappier than it did on the older ones, and they all used pretty much the same CPU. 320M really gave the UI rendering speed a clear boost.
Either way, I wouldn't worry too much about this as long as Apple implements the newest revision on ULV SB where the GPU doesn't run at 50% of speed of the LV one.