Sensitivity and efficiency are not the same thing. Sensitivity is acoustic output relative to voltage, usually specified as 2.83V, efficiency is acoustic output relative to power, usually 1 Watt. (If the load is exactly 8 ohms, then the number is the same.)
With two drivers in parallel, efficiency increases by 3 dB, and sensitivity by 6 dB. That is at low frequencies (relative to driver spacing), where the two drivers can be considered to act acoustically as one.
With four drivers in series-parallel, efficiency increases by 6 dB, and sensitivity increases by 6 dB. So yes, efficiency increases each time you increase the number of drivers i.e. +3 dB each time you double the number.
Those are "all other things being equal" / theoretical numbers. You might like to download something like Unibox and have a play with it (
http://audio.claub.net/software/kougaard/ubmodel.html). It will give you calculated figures for sensitivity and efficiency for various driver configurations.