I dunno. 40 seconds/20 rounds of extra attacks. Let's see.
20*6=120, times 120-ish damage per attack vs players, 14400 damage. Over 40 seconds. I dunno. Unless you're looking to carry your Antei of Formation from somewhere near the end of the first Keiiken to the beginning of the second(for a better quivering imprint or whatever), the trade-off likely wouldn't be worth it. That much Constitution would also give you an obscenely long whispering wind antei of well over half an hour, but should you lose the affect by death or whatever, you still have to wait 2 minutes on top of the remaining prevention in order to put it back on. Now, if whispering wind/hakanai/antei of formation could be stacked like immunities(due to having shorter prevention than duration), everyone else would be up in arms.
I do think Antei of Formation being a little shorter in prevention would be somewhat of a self-correcting problem, if the endurance cost remained. You lose a huge chunk of endurance(hit/dam as well), in exchange for a short burst of increased damage. Of course, doing the math for the balancing point is not exactly something I'm going to go into after a long day of work/just before bed. :/