.....and in that case, measuring the resistance / force it takes to twist the bushing.....gets complicated, because the effect on the bushing is not side-to-side, as much as it is up and down....creating even less "bind"
Very true. HOWEVER, the fact remains that ANY rubber or poly bushing is still going to exhibit force that affects the wheel rate of the car in a roll condition. That force may or may not be linear, but it is still present. It doesn't matter if it's natural rubber, synthetic rubber, 58, 68, 80 durometer poly, Whiteline's proprietary unobtainium compound, or any other elastic substance, it still exhibits torsional resistance. If you REALLY want to eliminate that impact on wheel rate, the only solution is some sort of Heim, monoball, Johnny-Joint, etc. But, as has been noted, there is a tradeoff. By eliminating that elastic suspension, you have also eliminated ANY vibration absorption, and the result is increased NVH. For the truly hardcore, it's no issue, but for the street-driven car, it's probably not worth the aggravation. Can poly bushing work acceptably well in the lower control arm? Absolutely. Just lock the car down with massive bars, uber-high spring rates and damping curves, and they'll do just fine. I forget who said it, but: "Any suspension design will work just fine if you prevent it from moving."
I have personally been through three different designs of LCAs. Stock rubber, stock arms: They sucked. Limp as noodles, and the rear end could dance around like crazy. Adjustable poly/Heim: They sucked. After two track days, the poly bushings had destroyed themselves, tearing from the roll forces running through them. Adjustable Heim/Heim. Near-perfect. Pretty massive NVH increase (not as much as the poly motor mounts, though!), but absolute precision in force management, with an appreciably quicker reaction to load transfers. As it is, I think my tolerance for NVH is perhaps a tad higher than most. For the street-driven car, I personally think the best bet would be a nice, stiff tubular arm, with nice, soft rubber bushings. At least if you're going to be pushing the car around corners. For the straight-line crowd, go poly.
Right now, the only bushings in the car that aren't Heim joints are the swaybar bushings and the front lower control arm bushings. I understand MM is working on Delrin FLCA bushings, and I am patiently waiting for those to come out.