I know of racers that have used the poly bushings for most areas, but used the original or soft poly for the front leafspring/trailing arm mount. The reason was that as the axle shifts under cornering, there is a little twisting/torquing of the front mount and with very firm or non-deflecting bushings, the load is transferred to the chassis mounts and it cracks them. I have not seen this personally, but when I worked on Alfa's, we repaired many GTV6 front swaybar chassis mounts that were ripped out after the owner installed 'hard' bushings.
Unless you have a 'Watts Linkage" the rear end is moving in a very slight arc as it goes up and down. I don't know how much lateral movement there is with a Panhard bar.
Any thoughts on this Bob? Is it a real concern?