Decided to splurge for a better towing package knowing the Nor-Tech 390 is both longer and heavier than my previous 340.
Heard good things about the GenY Torsion Hitch from a fellow CC owner who owns and tows a Mystic. Says he has over 10k towing miles and loves it.

Yes that's what I've been told, same principal as the trailer torsion axles.

Your question / concern is valid.
Here's my theory; a NT-390 on a trailer w/ 1/2 tank of fuel weighs approx 16,000lbs.
GMC advertises their Class V hitch towing capacity @ 20,000lbs & suggests a 10-15% tongue weight.
I've never been able to achieve 10% tongue weight w/ any boat I've owned including my 34' Nor-Tech.
Lets assume I can, that would be approx 1600lbs on the ball on a floating torsion hitch so I'm not only under the receivers suggested max tongue rating of 2000lbs but I also have a hitch that is absorbing the load and dampening the leverage you have raised concern about.
The GenY hitch has quite a long 2.5" x 2.5" solid shank w/ 2 hitch pin options, I believe it's in as far as it will go in the pics but I'm not 100% sure which hole, I just installed it and pinned it for the pics.

DISCLAIMER: I'm not a Mechanical Engineer, so this is just my hypothesis

^^^^^^^ I think payupsucker was talking more about the side to side leverage, kind of the same reason 5th wheel/ goose neck trailers pull better. For that I assume we will have to see when you start pulling the boat.

Understood, I didn't interpret it that way nor can I answer w/ any confidence.

In the past swaying side to side or the tail wagging the dog as I refer to it has never really been an issue for me but I've also never owned a boat/trailer combo this long and heavy.
Upgrading from a 2500 to a 3500 DRW increased trailering stability quite a bit.
I find that as long as I'm accelerating past an 18 wheeler I rarely experience sway vs on cruise control and an 18 wheeler blows by me.

Gooseneck / 5th wheel towing certainly offers more stability and towing capacity.