Not stepping on anyone's toe's here BUT If you are getting such a serious problem then IMHO you need to be looking for a fault, Zeds are not that bad in the stock form, if you push them when running OEM setup they are a bit soft due to the GT design but not as bad as yours seems to be, I'd be looking at bushes, broken drop links or failed shocks. if a shock has failed or a front spring is broken that will give you the sort of weight transfer that may give the lift of oversteer you seem to have. This applies to a broken drop link or bush something that will suddenly shift during the transition from on power to off power.
Zeds don't step out at 40mph on a corner if you lift just off - or they shouldn't.
Good luck with it but I'd find the issue before upgrading anything and then personally I'd start with the ARB's