(No postwork. Click to enlarge)

As usual, things are not that black and white.

First, a high polygon count isn't a sign of quality, as was amply demonstrated by MIKI-1.
M2 for example has lot LESS polygons than M3, still he's undoubtly the more realistic mesh.

But there are limits. Polygon smoothing and displacement can only be used with Firefly, and I personally try to avoid using Firefly as much as I can because I have better things to do than waiting hours for a simple still render to finish.
I also prefer to have "real" modelled in details, and when you want to portray an older person you need more geometry to create realistic wrinkles, etc, and here is where the Unimesh can shine.
Actually modelled details just look better than displacement maps IMO.

Posette has a good body with a very nice mesh layout, but what she needs is some proper kneecaps and maybe at least one or two-hundred more polys in her face, especially around her eyes, nostrils and mouth.
V2LO has even less Polygons in her head, but they are distributed differently. She has more in her face and less in her skull, that's why she looks more realistic than Posette.
V3RR is even better, in fact you hardly can tell the difference between her and regular V3 in most renders, while she still renders as fast as Posette.

Posette's default joints are pretty bad but can be easily improved.
Yamatosan made new injectable joints for Posette for his LinLin character.
And if you want to see what's really possible, Xaa over at Daz made a custom rigged Posette that easily outbends V4.
(If one wants to be really adventurous, she could even be regrouped to use V4's joints which would mean she could also wear V4 clothes.)

One other problem are Posette's morphs. There are a few nice ones around, but a lot of them are just not as high quallity as the DAZ ones.
She would especially need a set of new expression morphs that equal MIKI or V4's in quality.
(And here the low amount of polygons in her head again becomes a problem)

Soo, I think dphoadly did a fantasitic job, and it sure will Posette make a lot more usefull than she already was.
So many thanks for your great work !

But if one really wants to "replace" V3 or V4, one has to match the realism we already have with the latest DAZ and EF offerings (Especially  MIKI-2).
And that means that the remapping would be just the beginning.