All the above is great advise.
BUT......I would advise playing with your tyre pressures first and getting a full Geometry set up. How many miles has it done?
It could just be the suspension components/bushes etc are a bit tired to.
The Zed is not the most planted car in the world, but it should be better than your describing. I have stock suspension, although very low mileage, and quite light Forged 19" wheels with 275 RE050A's. With 420bhp I still have to try quite hard to get the arse out mid-corner in 3rd gear and above. Don't get me wrong, it could defiantly be better but what your describing sounds like something is not quite right.
Just a suggestion, and might save you £££