A few years back, I was engaged by a local manufacturer to do some development work with them for a range of bushes specifically for Minis, so I've been able to see Poly from both sides. I spent around a year working with them and we tried a few different compounds and other things to get the best we could from them. None the less, despite going in to this with some enthusiasm for the product, I did come away from it with a considerably different view on Poly and frankly, I don't like Poly anything on Mini Suspensions.
It would seem Keith Dodd from Mini Spares shared a similar view, this is info on Poly Bushes that was on their web site:-
"Aftermarket bushes for suspension and subframes have been a topic of conversation for some time now. Many folk have become obsessed about fitting brightly coloured plastic (polyurethane) bushes in place of any rubber bush they can find in the belief they will improve the performance of whatever bush has been replaced. Whilst it is true that polyurethane is harder wearing - and therefore longer lasting - than rubber in many applications it is not necessarily the best material for the application every time. Polyurethane has greater abrasion resistance and is stiffer for a similar hardness, is much cheaper to produce (and therefore sells at lower cost) and can be produced in a wide variety of gaudy colours.
However, it does not posses the elasticity, vibration absorbance or deflection tolerance of rubber, pretty much why manufacturers use rubber on production cars instead of the cheaper polyurethane. A case in point (and what brought this subject to the fore) is bottom arm bushes used in competition where spherical rod ends are not allowed. Increasing the caster angle to improve handling distorts the inner bottom arm bush dramatically. The more caster angle used, the more dramatic the distortion. Whilst a rubber bush will tolerate this to a certain extent, a polyurethane bush will deform and stay deformed - adversely affecting the bush's performance and eventually failing in an unwanted - and possibly dramatic way - by falling out, split into several pieces. Ideally a rubber bush with a metal sleeve in it to reduce the quantity of rubber in the installation to improve stability and control is the way to go; especially if that metal insert is offset at an angle in the rubber to counter or reduce the deflection applied by greater caster angles than standard. "