Not necessarily. One could argue based on epa numbers that the D models benefit from more braking Regen, hence the higher numbers. You'd need strict Watts per mile on the highway at certain speeds to just if the D was actually more efficient.
My WHPM on the 2016 90D is 275 on the highway vs. the 310 my 2012 P85 was getting But that's not particularly a fair comparison ... would need to compare with a recent S90.
For me the AWD is a big bonus even on dry pavement especially when accelerating from zero while taking a turn (e.g. pulling out into traffic). The additional efficiency is just a nice bonus.