I have no experience with the tail naca vent since the previous owner of my airplane closed it up.
However, I've heard from a number of people that have the flapper valve that they eventually went with a servo to positively close the flapper for pressurization.

As for capacity,  I haven't heard any LIVP owner say he had too much cabin ventilation.   ....and I've heard a lot that went to a/c in operating conditions that would be completely tolerable in a cessna.

Personally, I've found that ground ops are the biggest problem relative to ventilation.   having the door cracked open an inch or two is enough for me though.  I'm thinking of making a little jig to hold the door open when one set of latches is hooked.

I think if I were to do it over again, I would make a big fat door, maybe 4" x 4" or bigger, to stick out into the prop blast and catch some air when on the ground.   some engineering required.

Colyn