I tested a change in rear strut assembly on a gen 4 lib sedan because the shocks seemed a bit tired.
I compared the STI rear set with pink STI springs and the Bilstein tuned for STI vs the GTB black springs and GTB Bilsteins
For REAR The Pink STI spring vs Black had same no. of coils, same height, same spring coil diameter.
The actual Bilstein was softer for the STI than the GTB.
When installed the ride height was identical however the GTB rear was a little stiffer in the shock dampening. Using same 20mm solid rear sway bar.
This seems unusual but is it possible the rear STI springs only received a colour change but are essentially identical?
The FRONT I can confirm are different but still only lower the front end by 7mm. The lower portion of the coils are closer together for the STI spring but the ride is significantly more harsh. The handling is more controlled by the sway bar and a 22mm upgrade does it's part. The Bilstein's are supposed to be tuned for the STI suspension however there is no change in feel in practice if retaining the STI spring.
Reports from some drivers are coil overs tend to be stiffer (although adjustable). With the condition of all our roads, I wouldn't dare going stiffer or even more than a 18 rim and less than 40 profile.
In conclusion, think hard and research if deviating from the GTB setup with Bilsteins and black springs. Suspension needs to do its job. Perhaps PINK is not worth the change unless you do plenty of track work.
It also seriously hurts when you see a tyre stuffed because of poor road surface or even worse a rim