Hi nosmonkey - As you may know from reading my posts, I am one of the few people here who is concerned about the transmission of noise, vibration and harshness into my car's body. I am running 15" wheels, which means I have wider sidewalls that most folks here, since 16" wheels seem to be preferred. I have a strut brace across the front shock towers, but have not otherwise done anything to stiffen up my car.
I've done polyurethane bushings in an e34 and in an e39; they make a distinct difference. In doing so, I know that the stresses that are not being absorbed by the rubber bushings don't disappear, they are instead transferred elsewhere into the structure of the vehicle. Those vehicles can handle the stresses.
My view is that the coupes flex a lot because of the way they were made. For this reason, the improvement that would be obtained by using polyurethane bushings in a coupe would be limited by the fact that other parts of the chassis of a couple will flex in response to the increased loads. And, we don't really know what effect these increased loads will have on the welds that hold the car together, and the sheet metal that comprises the structure of the car, before failures start to occur. Stevehose's thread on the rear shock tower tear, for example, had an influence on my thinking, as did the fact that folks have found structural issues around the emergency brake mount in the transmission tunnel and the fact that Coupe King sells a differential reinforcement kit to repair tears and/or fortify the metal around the dif mount.
If I had taken my car down to bare metal and had a metal wizard rework the welds and strengthen the door sills, I might look at this differently. And I think newer model cars can accommodate more stiffening because the metals and manufacturing processes were more consistent that those used by Karmann when building our cars.
All that said, I like the W&N solid rubber bushings, and would recommend them.