KPI sets your scrub radius, gives some wheel raise (in the same way as caster), gives some camber loss under compression and lowers your static roll centre a little. The main direct effect is on your srub radius. Fitting wider tyres or different offset rims will also change your scrub radius.With KPI set up to the max the steering feels more responsive, seems to give better feedback.
However the front does feels kinda softer (i don't know how is this possible). But i do know as an offroader sometimes we set up the dampers more an an angle (less upright) to get softer damping. Probably because of this and correct me if i'm wrong as per what i digested on the forums. More KPI increases the roll centre slightly. So i don't know if that could have contributed to the "softer" front feeling.
But what i don't like is that in the initial phase, entry into a corner it feels "precise", then mid phase it just washes, ploughs understeer.
End of the day i'm still curious how much roll does KPI plays in geometry/ alignment setup before i swap the top mounts around and dial in maximum castor. What was initially holding me back from the idea of dialing in more castor was the fear of fouling up the fenders as i'm runing 215/45 R17s (by right it should be 205/40 R17 by rule of thumb).
Jim, thanks for your feedback.I've tried minimum and maximum KPI on my top mount. Maximum gives more scrub so more feel but more kick back as well (bad). Minimum gives less scrub. reduces kick back and gives a lighter feel (sometimes not enough feel). With a lowered car running wider track I believe the sweet spot is between the minimum and centre position on the top mount. If in doubt, just go with the centre position.
As an aside, when I first modified my strut tower to further increase the caster, I thought I'd gone too far and stuffed the handling. Tweaking the ride heights and damper settings made everything good again. Your mid corner understeer could be something else.