are there any real aftermarket parts for the Sky? I know the platform is the same as the Solstice so maybe the question should be is there any aftermarket support for the Solstice?

 

To my way of thinking polyurethane have their place in the replacement market, in general when rubber parts no longer exist. Rubber is vulcanized on to a metal component, a shell of something that supports it and flexes when the parts go through motion. Polyurethane does not flex and in many cases the work because they move or rotate with in the shell that is left when the rubber is removed as part of the installation procedure. However rubber also compensates for misalignment issues on two mating parts, that polyurethane can not do, which may cause binding, and if the shell is rusty when cleaned out of rubber, that will wear on the polyurethane cause life problems and wear.
The only advantage I can see to using Polyurethane vs rubber is if the stiff and non compliant suspension has a performance application where life is not an issue. Or as stated earlier, a part is no longer available in rubber and the only option is polyurethane. I personally am not a fan of them, although I have used them when rubber did not exist any longer. That add a lot of vibration to things.