I still need to replace my poly beam bushes with solid ones, But I couldn't get the poly ones out (yet) with the beam still underneath the car.
Although the poly bushes seem very hard I have the feeling that for the application they really aren't that hard. I once saw a 2wd Cosworth with these yellow Powerflex beam bushes launch and you could clearly see the rear wheels move forward inside the wheel arch. This can only be because of the bushes allowing this. The car did have a 6 degree beam, good diff and semi slicks fitted, so very good grip. The beam moving forward when accelerating isn't such a big problem I suppose, but during hard cornering it will also move, probably changing the toe of the rear wheels which isn't good.