My personal flex considerations in choosing a boot is based on my skeletal, muscular capability to flex a boot boot to its forward flex maximum, or in my case to what point the resistance builds up just before the maximum. Every boot, besides its industry mandated flex index, build up in resistance before reaching said maximum(where it will not flex forward anymore), in a somwhat constant manner or increases in its resistance, so to speak.
In real world boot recollection (my opinion), the mono-block Technica XT24 was constantly stiff, through its full foward flex range till it reached its maximum flex- at times for my weight, leverage/muscular capabiliy could not reach its maximum on very very cold days- the shell would stiffen up considerably.
Whereas, a Technica Diablo Race 130 AND Nordica Doberman Pro 130 felt different, in terms of stiffness but were similar is the WAY they stiffened up in the foward flex range. They both demonstrated an increase of stiffness as one flexed forward vs. a constant stiffness throughout the flex range as mentioned in the XT24.
I currently use the pre Falcon Salomon LAB boot- the Salomon LAB SX2 boot with the hexagonal holes on the outside of each boot. It is a two piece boot like the Nordica Doberman Pro 130 and the Technica Diablo Race 130 however the overall forward flex is very soft, almost intermediate advanced in CONSTANT flex, but is very rigid laterally and very rigid to theh rear. I prefer them to use for the skis I currently race on- a 3rd generation cap ski: 2006 Salomon LAB SL at 165cm.
In summary, what I am trying to express, is that besides the ski/binding/plate collaboration. Equally important is the ski and boot interface as related to boot stiffness...with the focus on a particular skier's interpretation of flex index. That interface is so delicate, I'm afraid to say- it is truly mind boggling to recall how I simply slapped on my Head 360s, Marker Simplex explodamatts and Lange Pros without thinking or testing even 10% of what I mentioned in the previous paragraphs.
Good luck foghorn.