this is actually quite difficult.
on the front, the horizontal bush is easy to change, but wouldn't gain that much from being changed without the vertical bush, which because of its design can't be made in a "poly" material. Actually, a harder horizontal bush could shorten the life of the vertical bush, but that's no real problem.
The lower wishbone is designed to flex, with the rear of the wishbone moving across the car whilst pivoting around the front bush. This increases comfort ( the front wheel likes to have a low stiffness in the front-to-rear plane ). So, you can see that the front bush is already quite hard - it would be the rear one that you'd need to improve.
On the Lotus Carlton, the vertical bush was re-made in a harder rubber.
on the rear, there are two sets of bushes to consider.....
subframe bushes.
some gains to be made from changing to stiffer donuts I think, but at some cost to comfort. Probably worth noting that the Holden manages to dispense 400HP with no rear end bush problems.
wishone bushes.
Don't change the wishbone bushes for poly ones. simple. The geometry is such that the inner bush flexes a lot becuse the camber/track correction bar needs it to. Stiffer bushes will prevent the wishbone moving properly.