Russ, due to something Clem asked me for, I have been investigating rubber bushes and how they SHOULD be manufactured. I will explain the two processes, so sorry if this gets a bit long-winded and technical, but somebody may get some use out of this and I think it might put your mind a little bit at rest on your question.
The cheap (and incorrect) way of making these bushes, and unfortunately also the one all local manufacturers I know about use, is to simply make a jig to hold the two yubes in position, then apply some bonding agent to the correct areas, and then insert the rubber (rubber or EPDM is softened with heat and pressed in, poly is mixed and poured) into the gap between the two tubes. The assembly is then placed in an oven or autoclave to cure. Simple and easy.
The problem is that rubber, and to a lesser extent polyurethane, shrinks while curing. So you end up with a situation where the bonding agent is under extreme tensile stress, and it invariably fails pretty soon.
The right way to do it is to make a seperate mold, with sizes very carefully calculated. The whole mould is coated with relaesa agent, the rubber or poly is inserted and cured, then removed from the mold. The sleeves are manufactured especially with rough surfaces where it will contact the rubber. Now the inner tube is cooled in liquid nitrogen, and the rubber is heated, then dropped over the inner sleeve. Then the process is reversed, with the rubber and inner sleeve assembly chilled in liquid nitrogen, and the outer sleeve heated and slipped over.
If the calculations for the mold were done correctly, you now have an asembly where the rubber is under pretty serious compressive stress, and this is what holds it all together. No bonding agent.
Thus in theory, if these fancy poly bushes have enough interference with the hole in the suspension arm, those bushes will not move, ever. And you have gained a little bit of flex because your rubber section is effectively thicker due to the omission of the outer sleeve (although probably not over a standard bush because the inner sleeve here is much thicker.
My major concern is with the polyurethane itself. I have been told that poly doesn't handle high-force vibration well, eventually disintigrating. And this exactly the type of loading expected in this application.
So personally I prefer drop-boxes or X-links or some other method to recover the caster angle without having to move away from concentric rubber bushes.