I used to have Massive dogbone bushings in my SVT, just keep using it and it will break in. I made the mistake of installing it in the winter when the poly was hard, I thought the car was going to fall apart. I tried it again in the spring and by summer it felt near stock. Sometimes there were a/c vibes at idle, but it just feels like an old auto Escort at idle without the a/c running...
I wore out the Massive bushings and I now I have "softer" Energy Suspension bushings that feel like already broken in Massive bushings.
Foam, nylon... soft washers would be destroyed as soon as you go down the street. I always wondered if the metal sleeves could be made from Delrin, and would it last and make a difference in vibes?