Gene, you would lose some of that theoretical efficiency boost in a (passive) speaker because the crossover will compensate for baffle step losses (even if it's reduced due to center speaker boundary gain) via resistive attenuation.
In a 3-way you can avoid said resistive attenuation in that midrange area by crossing the woofer right where the box starts to lose efficiency. You can also optimize the midrange driver parameters themselves to cover a shorter bandwidth with higher efficiency.
As far as displacement goes, to hit 100db @ 100hz you need 76 cm^3 whereas to hit 100db @ 300hz you need less than an 8th of the displacement.
Let's say you've got a 4" driver (75cm^2 of radiating surface area) - it need about 1.3mm of xmax to hit that target at its lower limit frequency around 300hz.
Now let's say you've got two 6.5" drivers (133cm^2 of radiating surface area each). To hit the 100db target at 100hz (where such SPL demands are probably more common) those two 6.5" drivers need 2.8mm xmax.
It seems to me that as far as displacement goes, the 3-way's mid has the advantage over the dual midbasses, because it's asked to move less overall at the same SPL at the bottom of its passband.
So the 4" dedicated mid does not necessarily have the displacement disadvantage. With the right engineering it really should not have the efficiency disadvantage either, as it can get away with around 3-4X the FS and less box size limitations for a sealed midrange. You can also get away with using metal cones like aluminum without the same disadvantages, because a 4" driver's cone breakup would be a good bit higher than that of a 6.5" driver.
Additionally, the 4" dedicated mid probably allows a higher crossover frequency to the tweeter for reasons mentioned in the article, which improves the tweeter's power handling.
The dual 6.5" woofers do have an advantage as far as heatsinking area is concerned, but are also asked to cover almost 2 more octaves, where a lot of power is centered.
Finally, it means you can use 8" sealed woofers in the lower mids and upper bass, which will blend in better to subs than vented 6.5" mids.
*shrug*
Also, let's say your center is about 9 feet from boundaries. That sound still arrives at your ears within 17 ms of the direct sound. It won't affect timbre/imaging but I would not suggest it does not affect our perception.