A very personal view from a german:
Definitely important:
PDCC & Rear Axle stearing (--> huge improvement in handling)
21 inches: The car feels much more 'planted'. Moreover, 20-inches-tyres are designed for economy, therefore breaking distance is considerably longer (I have bought mine w/ 20...
Most important: as almost everybody here is stating - it is by far the most exciting car I have ever owned. Right now I can not imagine of any substantial improvement - and that has never happened to me before.
As to the bugs: yes, there are some - and some of them stated in this thread my car...