What's the goal here?
Efficiency refers to a speakers ability to put out more volume with less power but it has no bearing on how "good" a speaker sounds. That's like looking for cars based purely on their mileage rating.
i.e. There are many efficient speakers but their sound does not float my boat. And power is not the expensive thing it once was.
But, efficiency is one of the three main critria in designing speakers. to wit:
1) You can have efficiency.
2) You can have deep, extended bass.
3) You can have a physically small enclosure.
But, the law of physics says you cannot have all three. Two, perhaps, but not three.
So, I guess you might find out from your mentor what he's shooting for.