From what I've read in these forums over the years, is that in many members opinions the STi struts, particularly Ver 5 and Ver 7, are much more on the compiant side than your typical Japanese aftermarket products.
I believe most of the STi struts being grouped together here have multiphased damping. The force velocity curve starts off with a steep slope for good low speed damping (body control) and then tapers off to a gentler slope for more forgiving high speed damping. Struts with a digressive curve typically have less high speed damping than a strut with a linear curve providing similar body control (low speed damping).
On paper, the posted shock dyno curves for some of the STi units depict curves, that in theory, should provide decent ride quality. However, due to various flaws, design traits, and other factors not yet fully understood, the STI inverted monotube stuts end up providing a ride quality that does not match it's valving. Put that same relative valving, giving the same damping curve, and hence the same amount of high speed damping, in a non-inverted twin tube design and I think you'll end up with a much more comfortable ride over the long term.
If your struts are knocking/clunking, then they're not functioning properly. The noise most likely comes from the lower end of the strut cartridge making contact with the inside wall of the lower housing. When this happens you end up with metal rubbing on metal. The additional friction alters the damping and causes stiction.