Driver efficiency vs. overall speaker efficiency

Twl is correct.

Though, one should question the heck out of companies who use drivers of 88 db/2.83 V sensitivity and then claim the speakers are 94 db/2.83 V sensitive. I know of one famous one that plays this game egregiously.

You are correct about the definition of efficiency, Gregm. The term we are actually discussing in this thread is sensitivity, but I didn't want to interject it until you so wisely brought up what efficiency actually is.

My perspective is that the box should add maybe a couple dB to the sensitivity of a driver. And, that is the best you can do, using a driver such as a Lowther. When you begin adding crossover components, the senstitivity goes down, and the impedance curve begins to dance around, both of which make the speaker more difficult to drive. Think of crossover components as speed bumps for your amplifier.