Polyurethane bushings are not all the same. The hardness varies widely from on company to another. Some are really soft and deform under use. Keep in mind that polyurethane is not like rubber. Once it deforms it will stay deformed. I used some soft polyurethane bushings in my rear control links and they actually ovalized in the course of a year. The stiffest polyurethane bushings seem to come from Vette Brakes and Products. I've never had a problem with their bushings. At this point they're the only ones I use. Now it's just a matter of getting all the other ones out of the car.
If you have over 100,000 miles on your C4 you're probably a candidate for polyurethane bushings. They're not as easy to install as some would have you believe but it's not all that bad. You can do them on one of those weekends when nothing else is going on. It's just a matter of taking your time. New bushings may very well be the least expensive way to revive your old tired suspension. I think it's worth the effort, just don't believe all the hype that you're suddenly going to pull 2.0 Gs in a corner.