Well a year after starting this thread I finally have the car settling down on the FSD's.
I've driven a couple of hundred miles on them so far and I can still feel the car finding it's final height and posture.
So far here are my thoughts:
I had factory sport on the car before with 100k miles on it. I've driven brand new factory sport as well as just about every suspension system from street to track to rally. The factory sport was a great option on these cars, mainly due to the additional weight of the V8 and the extra width, which changed the way the mechanical grip was laterally used.
We decided to keep the factory sport springs as after inspection they are still in perfect shape and hold the car at measured equal height all round. Also didn't want to lower it, for many reasons!! So it was those wrapped around the FSD's.
Initial impression was "woah, this feels like I'm on a magic carpet!". Probably the first 50 miles were like that as the shocks and springs searched for their new happy place. I could feel the car settling all the time. It went from "magic carpet" to "just magic" after around 100 miles. The heavy front end has never felt pointed on any V8 C5, to me. The car now feels like it wants to go where the corner is going. What a revelation! Roads I've driven hundreds of times have changed totally from a drivers perspective. Freeway on and off ramps have become fun parabolas instead of constant understeer battles.
On the freeway at @#$mph (!) the car rides like a comfort suspension C5 or actually better. Expansion joints, on CA's crap roads, are a mere blip now where there used to be need for spinal surgery previously. Again, roads I know really well are being overthrown by the good manners of this chassis now. However, change lanes quickly and the car seems to read your mind, a little like DRC does. I know very well how these cars settle when they have been moved off line quickly and this system has removed almost all of that rear end doubt. It's actually slightly addictive to keep feeling how settled it feels as soon as the steering input is removed.
I'm going to give it a few hundred miles and then get everything torqued down again. Then I'll go find something twisty to drive on......
All in all I am so pleased I went with this setup. I really do think Koni have hit on something here that is the better of both worlds if not actually the best of both worlds.
Thanks all for your thoughts and input on this.
So far so good.