there is no doubt that the poly bushes tighten things up and gives the car a better response to the road, there is way less movement or give in the bushes and this does transition to the cabin, i have had this experience in other cars and will be putting a full poly kit in the Zed as soon as i can.
again like a lot of upgrades it is a personal thing, for someone who likes the Zed as it is and likes how it handles then the choice is to go for OEM. If you are like me and you want to get more from the car handling wise and like to drive around corners rather than float around them then go for poly. for me i hear everyone say that the Zed handles great on corners etc but i disagree, i think there is way more the car can give with the right setup and that is the route i will be taking. Anyway, just said i would give my tuppence worth ;-)
Dave