The roller spring perch improves handling by allowing the base of the spring to remain level compared to the top of the spring throughout the articulation of the suspension. The stock perch is too stiff and so when you go over a bump, it essentially stays in the same orientation regardless of the angle of the upper arm. This means that the outside of the spring becomes much shorter than the inside when the upper arm moves up and down. This leads to variable spring rate (and not in a good, predictable way). The spring wants to become "c" or backwards "c"-shaped and causes the shock to bind (shocks work best up and down, without any sideways force on them), wearing it out prematurely, too.
When I changed out my old perches to roller perches, my old perches remained at an angle. I could only rotate them in a vise with considerable force. The suspension reaction is much better now, and the car handles much better than with the old perches.
As far as bump steer (which is caused by the lower control arm and the tie rod moving in different arcs, which pulls the toe in or out as the suspension moves up or down), the roller perches won't help any as you would still have the two arcs. You can get a bump steer kit from Pro-motorsports or Baer which will help, but you really need to measure the bump steer (or have a suspension shop do so) before throwing money at the problem. You may have other issues (like the strut rod bushings mentioned already) or other binding going on. All vintage Mustangs have some bump steer with the stock suspension, although the kits can minimize it some. Really stiff springs (like HistoricMustang uses) prevent the suspension from moving much and thus minimize bump steer. Like he says, this is fine on a smooth track but may not be so enjoyable on a rougher street environment.
Any time you take a compontent with a bushing that can bind in suspension travel and change it to a roller component, you free up that part to react in a more predictable, smooth way. You may find that you need a higher spring rate, though, as all the binding sites were helping to slow movement down! Opentracker stuff is excellent and he's a great guy/company to deal with, too! Ask him questions and he'll suggest the right things to you, even if it means you won't spend as much money with him.
Daniel
