If the car is understeering I would stiffen the front not the harden the rear.
Personally I think the GGR tightening kit is better than poly, or solid bushes, having tried all of them.
The poly diff mount makes a world of difference on it's own, without doing any other bushes to poly, yes they really are that shit.
The 22mm adjustable bar I made makes a awsome difference as well, but would only advise it if adjustable suspension fitted, doesn't need coily's just so long as adjustable.
tabetha