A stiffer boot is like power steering, it responds well to small but accurate imput. This can be a great thing if the skier's movements are accurate and a bad thing if they are not. While beginner skiers make gross balancing adjustments taking longer to detect and correct imbalances, experts want quicker and more accurate transmission of impulse to their skis. A stiffer boot, aligned properly, will aid balance recoveries as well, when the skier does get off kilter. A stiffer boot will need less movement to transmit the same amount of impulse as a softer one. A stiffer boot which limits ankle flexion requires the skier adapt his/her flexion movements to remain in fore/aft balance by actively breaking at the waist and reaching forward with the hands when flexing deeply. Remember there are only a couple places the body can flex forward, the ankles, the hips, and to some extent the spine. If one is compromised the others need to pick up the slack or the skier will be relegated to the back seat.
A stiffer, higher performance ski will be complimented by a stiffer boot, whereas a softer sport ski will work better with a like boot for that level skier. A stiff ski combined with too soft a boot, will compromise the skis performance.
As I have said many times before, the stiffer you go the more important good alignment becomes. The boot should place you in your optimum fore/aft positioning when standing cuff neutral.