I've had 3 sets of Avon ZZ5s on 2 different cars and I seriously doubt that they are what is making your Zed feel squirmy. Unless, as Mr Wilks says they have been mounted incorrectly. My Zed is fitted with a full 18" staggered set of ZZ5s and I am running the handbook/door sticker recommended pressures of 2.1bar/30psi front and 2.3bar/33 psi rear. I've never felt the need to raise/lower these pressures. Also I had Hunter 4 Wheel Alignment done when I changed to 18" rims shod with the ZZ5s. My car is an SE but is fitted with M - Sport springs. All other suspension components are standard. I've never had any issues and I know it's a cliché but 'it handles like it's on rails'...not a hint of squirm ! 
