It's better than the PP springs for sure drove both mine on OEM PP springs and one on OEM PP shocks and Eibach Pro's and the difference in body roll is big enough to consider it been faster on road course / autocross setting without been harsh on street.
Changing springs without changing dampers is a sure way to completely mess the handling up. It is a terrible idea.
So terrible that Ford are selling this springs for the OEM dumpers ? As part of the ford performance catalog ?
Given stock setup is underdamped - yes. Of course Ford Performance will sell springs (note it's UK - in US they don't sell it) - why wouldn't they ? 99 % of people installing lowering springs do not that to improve handling. They want "lowered" look.
This ones at least based on my short testing over my usual test looks seems to improve handling and make the car more fun to drive as well. I think Eibach sell them in US as well here for example:
you don't want the Ford cut-down bumpstops (crap) nor the 'adjustable' blue ones from Steeda. Eibach, and cut them down with a butter knife. Ok, hobby knife.
Where are you getting your initial spring perch height numbers? Are you just running the perch to contact? Is that even a rational ride height with the piston that far down in the shock body? I would expect several (10-15) turns of preload to get to some nominal ride height measurement.
I didn't know the bumpstops were that big of a deal... I thought it was just protection from bottoming out and damaging the shock... Am I missing something? Or am I on the right track, and it's just more important than I thought?
The Ohlins and the Bilstein seem to be separate shock/spring mount rear locations, so I would assume the spring rates would be a direct comparison to any replacement rear spring. I get that changing the location on the control arm would change the leverage, and therefore the spring rates wouldn't necessarily correlate to the same effective spring rate. It seems counterproductive to move the spring mount out to the weaker shock mount just to have "true" coilovers in the back.
unfortunately car guys have decided to use them as part of the spring. I am of the opinion that they should only be used as a last ditch means of decellerating the wheel assembly before hitting mechanical limit. That view is in the distinct minority however.
Yeah this is how I see them. If your springs are traveling so far as to bottom out during normal (race included) driving, that seems excessive. Plus I would expect a bumpstop to effectively act as a drastic change in spring rate, upsetting the car. I could see them hitting the stop when there's a big pothole or something, really causing drastic, uncontrolled movement of the control arm, but yeah, last ditch damage prevention