You're right, introducing an axial rotation DOF in each link will still leave the parallel 4 link overconstrained, but it will remove one additional overconstraint in the case of poly bushings at both link ends, without the NVH and wear issues of heim joints. From that perspective, a three link has the same issues. Just how much remaining deflection is required to accomodate roll in the normal plus/minus few degrees range is the question I don't have the answer to, but if the resulting roll stiffness introduced by the deflection is a low percentage of the total roll stiffness created by springs and sway bar, it will not be a significant factor in the handling. Engineering is the art of informed compromise, and what appears to be a compromise may be well-informed or not.
I completely agree that roll center, roll axis, roll steer, and instant center, and the variance of those parameters through the suspension range, are the primary determinants of the handling characteristics of a rear suspension design. For consumers of these products, if the vendor can't answer what those parameters are for a given design, and why those parameters were chosen, it is a complete crapshoot whether the design will "improve" handling. And to make the point I was originally trying to make, you can't determine these things just by counting links.
The good news for PT guys on a budget is that plain old multi-leaf springs can do a OK job on almost all fronts, with the primary compromises being static friction (which can be reduced by careful design) and unsprung weight (which can't be fixed, unless you go fiberglass, which has its own issues).
Yeah, Katz did some pretty good work at AME. Most of those old-line chassis makers have no serious engineering in their suspension designs, but the AME stuff is pretty good. As it happens, I just picked up the Tri5 Morrison chassis for my wife's 55 BelAir a couple of days ago. It'll never see the track, but it will drive a lot better on the street!
Good luck on your new company, sounds like you guys are pretty jazzed.
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