GRIDHEAD- some interesting points. There's a SRT-8 used suspension kit for sale in the states for £200 on ebay (didn't sell for £200 last time it was listed). I have considered this.. The one in Germany is considerably more expensive (relatively), but he may take a crazy offer - who knows!
I wouldn't touch any aftermarket stuff with a bargepole - i'd much prefer something that's been developed and tested by an OEM.
I am quite surprised you found the ride quality to improve by going up to 20" alloys. Reducing sidewall compliance typically does nothing positive at all, unless you were perhaps running something like a 70-profile tyre in the first place
.. The lowest sidewall profile i'd go to is 45, which I believe is the choice for the 20's anyway. I am tempted by the SRT-8 alloys, approx $2k from the states with tyres (+delivery).
I have found the auto-levelling dampers are crap.. the car 'buckaroo's' on the motorway when going through large dips so i'd have no issues losing that 'feature'. I have been equally disappointed by the low-speed damping, which also makes me curious how reducing the sidewall could improve things (as no doubt the medium-compliance the stock tyres have is hiding some of it)..
Last but not least, one thing that is bothering me about changing the suspension, is the difference in weight and balance of the car (being a diesel). I would assume the diesel engine is still heavier than the v6/v8 versions and thus would have different rated springs (or perhaps also dampers) than the petrol versions. Can anyone confirm if this is the case? Last thing I want to do is bolt on SRT-8 dampers/springs to find the front end is too soft/low due to the extra weight...
Thanks