The ST X coilovers are the same springs as the KW v3 .The ST XTA coilovers are a little different (front springs are linear) and better suited for daily/occasional track use.HKS was very highly regarded in the Mazdaspeed community. But being a single tube design, they will be a little on the stiff side. I do like that they offer the optional ADS cancellers.
I am bummed that KW does not make a V3 specific to the Si Sedan. I ran those on my Speed3 for ~6 years/130K. But you may want to check with a couple suspension shops on their experience with the non-Si V3 to see if that will fit your needs. Like HKS, I believe KW sells an optional ADS canceller. GL.
pmd youI have some ground controls for sale if you are interested in keeping the ADS light off.