personally i wouldn't recommend fitting poly bushes, they are very harsh and the ride quality will decrease.
In addition there is a possibility that they will damage the mounting points on the car. In the past i have fitted a set to a Mk2 cav, after 10,000 miles the chassis was cracking around where the bush joined the body, the reaso for this became clear when we compared the way a poly bush moves to a conventional bush.....
The poly bush was a two piece design with a stainless sleeve, it was designed to be done up so tight that all the movement was on the sleeve and not in the bush, the OE bush was designed to flex in the bush and not in the sleeve, the result was that the sleeve cut into the chassis leg as the metal was harder than the steel in the leg, it caused lots of hassle.
The Miggy bushes are different in design to the cav ones, my point is that the poly bushes are designed to move/flex in a different way, and that is not always the most effective way fro the design of the car