And if you want the suspension to be firm you should fit powerflex or bmw e34's white or green bushes while you got the front susp open
Just to clarify this for the op...
White or green BMW bushes have an offset centre pivot point iirc? and are still rubber and likely to fail in a similar life span.
So powerflex is the way for bushes as you say. They will, however, deliver a slight increase in harshness through the steering wheel though, but it's considered a price worth paying for more accurate steering and better life of the bushes.
So there is an element of "firming up" with bushes but this is a separate sensation to the that of the shocks and springs, in that the bushes resonate smaller high frequency bumps to the driver, where shocks and springs deal with bigger/higher frequency bumps.
Both have in input on comfort and ride quality, balanced against accuracy and better basic handling. Softer rolling suspension has a detrimental effect on tyre wear as well for more enthusiastic drivers, so worth pursuing in that case.