Focal Kanta No.2

That is an rather myopic view of engineering of any mechanical system. You're comparing three motor system which will in combination require more power than the single driver. The cabinet and baffle construction will be more complex and costly and the bass performance of each driver will vary due to mechanical acoustic properties as they won't have identical operating environments in the cabinets. You do get some advantages, but those don't outweigh the complications. Even if you increase cabinet volume, you introduce cabinet anomalies if not properly braced and damped.

Using an array of smaller drivers is a solution, but I wouldn't consider it optimal application unless the room size is small, which you had mentioned is your current setup. In my room, which is more than double what Focal specifies for the 936 and still significantly more than the Kanta2, going to larger cone diameter is the best route. In my case it works ideally with two eight inch drivers, but a single twelve could do as well. The negative to the twelve is wide baffle and cabinet construction to contain two eights are reasonable in engineering and cost. Right sized tools for the job is always best. 

It's not that myopic at all. I've heard the argument that all those individual sources can be identified, but the physics don't reflect that. If you're argument was correct, the 948's would require less power for a given output in the bass region, but the require more using fewer, larger, but otherwise identical drivers. The physics of the system suggests that the drivers mechanically couple as roughly described by line arrays theory and deviations tend to nullify, not amplify. 

The Focal 948 are slightly more efficient over the 936's, but cabinet volume in relation to surface radiation area is a bit less on the 948 as compared to the 936. The main reasoning to not going further is the efficiency for the mid and tweeter drivers are not any more capable in the 948 or 936. At 92db vs 92.5db, either design is at the upper end efficiency for typical cones and domes without resorting to horn loading or other techniques for the mid and tweeter. Focal hit the limit of those drivers first and in the bass could instead be tuned for deeper response in port tuning and lower distortion.

A good deal of other lines out there do gain significant efficiency in larger models as they shelve their mid/tweeter drivers down in level in the crossover in smaller designs as compared to larger. The flax cones are pretty light and achieve output similar to coated paper drivers, but obviously they have their own set of compromises. 

Now the line array theory isn't entirely effective in the 936 as the driver symmetry goes off in the upper end of their pass band. Since its  pretty low in level, the audibility isn't high and is given as acceptable in a design at this price point. Agreeably, this has more to do with the more complex bass driver arrangement in the 936 over the other models in the line. To make that coupling work really well, you require high constancy in driver and operating environment, which some designs to practice to good effect.

No, the most certainly are not more efficient in the bass region! Do the math. .5dB is just about the margin of that error so that increase in sensitivity isn't going to amount to a hill of beans. However, the 2.5 ohm impedance right in the middle of their bass pass band is going to draw more power. 

As for the line arrays theory, generally, coupling breaks down once wavelengths exceed the on center distance between drivers. That distance on the 936 is under a foot so they don't really break down as a coherent mechanical unit until 1000Hz. They're down about 18dB at that point. My guess is that's why you see some odd behavior from the bottom 2 as the top one tapers off more shallowly. 

It seems we're generally in agreement. In any event, I'd expect the Kanta No.2 to have better controlled bass considering the only difference between those drivers and the 936 drivers is a significantly more linear motor. 

My expectation is the use of the extra current use was to extend very low bass and the phase angle is moderate. Still, I would never suggest pairing a Focal design with anything that doesn't have good impedance stability. It is likely one on of the greater causes of complaint about the speaker balance as the wrong paring will change it.

Its likely the mechanical limits as the upper driver isn't experiencing this to the same degree. Since those are lower in signal amplitude, the relative mechanical impact is increased.

Granted the efficiency gain is in a typical margin of error, but I expect Focal gave it that slight nudge as its own measurements had shown it regularly having a slight advantage. I still believe Focal was at the efficiency limit of the mid or tweeter driver instead of the bass arrangement when designing the 948.

But yes, for the most part we are in agreement with differences in talking points only.