Hello Michael - unfortunately only "borrowed" a 400 for a week but owned a 2010 NA and a 2013 S. Had my first "spirited" drive after first service at the weekend on some Spanish mountain roads. It was phenomenal - the feel of the car is great - before the tyres properly warmed up I could feel it quite close to the limit and sliding a little but once warm and really pushing on there was so much grip. It felt so much more "planted" than my previous cars and the feedback much better - and I thought they were good! Took out a friend who had been in the S - she thought it more comfortable than the S - perhaps she was just being kind.
After the mountain roads some high speed motorway for 3 hours - again very comfortable and no fatigue at all. Even in sports mode it wasn't unpleasant at all in fact the engine noise is so sweet you want to drop a gear and hear the howl every now and again.
The last km of the journey was actually "off road" - had made arrangements to leave the car nearby but thought I would see how far I could get and it handled it really well! Not sure will make a habit of it though....
Being able to use the full rev range now I can really appreciate how quick the car is, first gear is very short but shifting into second and taking it to the red line in second and third you are into 3 figures very quickly and the sound......
Love the simplicity of the cabin - the heater is great, demist very fast. Really abused her over the weekend and will treat to a valet tomorrow and get some pics!