Jamie pretty much nailed the explanation. According to Chuck, the ploy/bronze bushings should ride almost as smooth as stock rubber. The truth is that this is almost impossible to verify, since I know of no one - myself included - that has ever taken a stock, well tuned suspension (i.e. no worn out bushings and shocks) and ONLY replaced the bushings to see what the effect was. In almost every case, LeShandleritis comes on strong and you go from a stock suspension with Boges and 18mm torsion bars to 21/22mm t bars, Bilstiens and the polybronze busings. So yeah, the new suspension is stiffer - but why? You've changed too many variables to be able to say the polybronze do or do not contribute to harshness.
On my recent suspension rebulld, I went with the polybronze in the front and the spring plates. It was not cheap. The reason I'll tell you when we're all BSing at the track is that I really liked the way the help keep the geometry in line and controls the tendency for the rear wheels to toe out when cornering at 9/10s. The reality is I have no idea what that means - I used them because I'm lazy. Functionally, they operate in the same manner as the replacement polycarbonate bushings - i.e. the a-arms twist inside the bushing rather than deform it like with rubber - then I didn't think there'd be much of a performance difference between the two. However, talking to Charlie, I was told to expect about an hour or so of work to fit each bushing. 4 hours total for the front. Screw that - for $240, they were installed in about 15 minutes. The rears were similar.
Oh, and I hope they do some of the cool keep the suspension in line stuff on the track. I'll let you know next year.
