Poly bushes do not produce a harsh ride! The springs and dampers are the key to ride quality. My car has lots of small changes to improve handling, and with Bilstien Sprintline dampers and Eibach springs on the front (standard rears to maintain ground clearance) it has a very good ride quality. In comparison to a Z4 Alpina it is far more comfortable on rural French roads.
My Powerflex poly bushes did not make any difference in ride quality, and If you have 17 inch wheels they stop the usual so-called 'Bump steer,' which is caused by the original gapped rubber wishbone bushes, which compress sideways in response to loads on the tyres - probably the best change I have ever made.
If you want to go the OEM route, use the bushes from an M3/M Roadster, which are almost solid rubber.
(See my bit on Stiffening a Z3 in the 'Articles' section)