This would be a good question to ask on the boot forum, or the instruction forum. Not clear that a turn begins with flexing the ankle, if that's what the sainted JH said, but in the opinion of some respected members here, yep, we're going overboard with boot stiffness. Keep in mind that modern technique is all about lateral movement, and even moderate forward flex boots have very stiff lateral support. Plus modern liners are much better at transmitting force directly from foot to boot. IMO, forward stiffness is more about skier's body weight, habitual speeds (both of which produce force), and style (mashing forward is considered old school, however honorable).
I'm 165. I've found that my racing improved when I dropped from 130 to about 122, and my recreational skiing is better at 110-115 than at 120+. These may reflect two different issues; racing demands rather precise timed foot movements inside the boot, which a super stiff shell actually slows by requiring more force, thus more time, and also racing can get very choppy/bouncy at speed, which a super stiff boot will transmit right back to your lower leg. So you spend more attention adjusting/surviving than turning. Our junior coaches actually sent around a letter asking parents to ease up on getting overly stiff boots for their kids.
For rec, a somewhat softer boot is better at absorbing shocks from bumps or variable soft snow and not transmitting every mechanical misstep back to the ski.
Obviously, if a boot's too soft, then it becomes inefficient at transmitting force to the edge. Just a question of each person's sweet spot. And last, modern skis have become stiffer, to compensate for having shorter running lengths due to rocker, so the boot has to match the ski decently. A super stiff boot on a soft ski doesn't work, although it may be preferable to a overly soft boot on a stiff ski.
My .02, others will disagree, I'm sure.