Like I say, you will really want the lowering springs though, as otherwise, despite the huge grip and huge cornering speeds, the angle the car will take them will scare you until you're used to it. When lowered, there is a LOT less roll and you just feel so much more confident to push !
You're better off replacing the rear ARB with a better one - the Whiteline 22mm is the usual choice. Dials out some of the understeer at the same time.
A full set of Defis will should be about £800 including fitting - the exact price will depend on gauge model.
M