It actually is an absolute direct correlation and proven through both computational FEA and measurement. You get a substantial area in increase when moving from six to eight inch cones and this allow substantially reduced excursion and pressure exerted on the cone to achieve similar amplitude at lower bass frequencies. So while the shorter cone is stiffer due to its smaller size, its generally a linear gain unless shape is changed, which it usually is in most cases. The force increase on the other hand is a non linear function as there are multiple factors at play, the pressure itself and increase in cone velocity and acceleration in air. A smaller driver will always requires more energy to achieve the same output as larger driver in low bass area. An eight inch cone offers about fifty percent more area than a six and half inch cone. Now as that cone gets larger, the ability to achieve the rates of acceleration at higher frequencies becomes overly challenging. Not to mention the dispersion beaming that also occurs, which a six inch cone begins to beam at around 2khz, maybe slightly less. I'd do believe there would be artifacts if the wavelength produced becomes larger than the cone, but I'm not sure if its of any issues at the sized being discussed.
In ending, there isn't a single part of the driver that wouldn't require significantly more buildup if wanted to achieve a similar lower frequency performance without distortion increase. It might even be easier to use Focal's own EM technology to achieve what your proposing, but at a notable cost. Going to a larger cone is simply much more cost effective and negative is mainly overall size and increase in baffle if front mounted, a negative if trying to increase dispersion width but can be addressed with baffle shaping.
