I've not seen anybody post up the consumption in bridge mode but there is known to be a discrete WiFi board which could be powered down and although the main processor can't be turned off the switch circuitry presumably could be or at least run at reduced power (some switches have a "green" mode). I imagine the power required by the CPU should also drop with the reduced load.
I guess it all depends on how skilled the firmware coders are and how bothered they are about your leccy bill...
So - probably unchanged then
Compared to having to fork out for a router to do the job the supplied hub should be doing the extra power cost is pretty small beer.