I've only been breaking TS but of the 4 so far, 2 had spherical which were fine ( although the rubber dust caps were starting to perish), 1 had rubbers which were knackered and working their way towards the subframe. The last had poly bushes which were all distorted. My educated guess would be from the fore & aft load caused by the rear wheel steering, but I obviously can't prove this theory. I will have a look at my own Powerflexed back end to see if there is any of the same. I drive like a nun most of the time but I do like to chuck it into a bend. The greater the speed, the greater the load transfer, the more rear wheel steer effect you get. It's the reason for the ball joint on the upper wishbone - it allows the rear upright to pivot on a vertical axis to steer the rear wheels.