Actually, my engineer's slide rule says the efficiency of the wireless should be about 86% of the wired charging assuming both are plugged into the same wall box. In fact the wireless charger could be designed to be more efficient than direct connection to the wall box except that would require the wireless charger to draw more current and some folks may be limited to around whatever current is conveniently available in their garage from a 220-240 volt service, probably 30 amps.
Speaking for myself, I would love to have the wireless charging available as a matter of convenience regardless of any efficiency tradeoff. I bought my 530e primarily for the silence and low-end torque rather than operating cost though I admit the ~$6000 discount after the federal tax credit and the state rebate compared to a 530i was certainly nice.
Incidentally, there should be little or no current running through the wireless charger once the battery has been charged.