If i use more than one of a given driver, how do i calculate the efficiency of the "array" ?

i've been wanting to build a pair of Karlson cabinets(say what you will about them) and for the HF/mid bit i figured i'd try something a little more unorthodox than a regular old mid + tweeter. in the Karlson's case, it might be easier to achieve an aesthetically pleasing solution this way anyway. plus it makes a method for me to achieve 8 ohm final impedance.

So i was looking around and it seems Tang Band has these 2-inch aluminum drivers that go from 330-20k Hz. it might be debatable whether or not they really achieve that low, but i was going to use them from 800 and up anyway. the woofer i wanted to use was 6 ohms. i figured 4 of these little 2" drivers(8 ohms each) paralleled woudl give me 2 ohms, in series with the 6 ohm woofer equals 8 ohms nominal. i plan to use a Zobel on the woofer, maybe the Tang Bands too.

thing is, the woofer is 95 dB efficient and the Tang Bands are each only 86, or 88 if i get the paper cone version. i don't feel like attentuating the woofer at all because i want high efficiency. if i string together maybe 4 of the TBs per cabinet, what sort of efficiency gain could i achieve for the collective four of them, if at all? how do i calculate or predict that? i guess it would sort of be a mini transmission line because i wanted to vertically stack them on one side of the Karlson box, which would just look like a segment of a line array i once saw built out of these things. not that i have any illusions of achieving the effect of a real line array, more like just a neat looking mid-tweeter stack. i know there is a formula in James Griffin's line array paper, but is that one that really only applies to LA construction?

thanks