Personally I would have thought it would be barely noticeable being as most Cosworth owners seem to have poly-bushed TCAs/ARB mountings already and are often running 17" tyre combos?

Someone once told me that polybushing on the front end just adds precision without too much extra NVH......it's usually once you start on the back of the car ( solid beam mounts, solid rear arm bushings etc ) that you start to really introduce NVH into the car and interior.....