The rate from the bushing is not really a big deal on our cars because we adjust for height / corner balance with the adjusters. The car doesn't really care if a little if the spring rate comes from a bushing or if it all comes from the torsion bar. Rate is rate for the most part.

I ran poly in my car for a while and it took me a year of trying to fix the handling of the car to figure out they were binding so bad they were adding a ton of spring rate. I switched to delrin, which took a lot of love to make sure they were aligned / free, and got my front grip back. But they were noisy and harsh. So I went back to rubber and the car is as it should be. Only rubber ever again for me.

Fun anecdote: in SCCA Street class (stock) some of the guys, especially in Miatas, use bushings to get an edge. They remove the shock, loosen all the suspension bolts (it's multi-link, so there's several), compress the suspension all the way, then tighten everything up before putting the shock back on. The bushing wind-up lowers the car slightly, and due to camber gain, gives a little extra static camber.