I think the understeer on sapphs is because of two reasons. The standard rear beam makes it understeer, as it toes in during cornering. I think Ford did this intentionally to make it an understeering car.

The second thing is the front roll center. At standard ride height it's not a problem, but when the car is lowered this increases the already present understeer.

Last year I replaced the standard sapph (non-cosworth) hubs on my sapp for 3dr cosworth hubs (higher roll center) and the difference was very noticeable. It understeers less and the car generally feels much more responsive. Later I also replaced the rear beam with a 6 degree one and the handling is really nice now.

With regard to the poly bushes, the most important to do are the tca to arb and arb to chassis at the front. And at the rear the big beam bushes and beam mount. If the rear arm bushes are worn the car will also handle strange.