I managed to make quite a bit of progress on this little experiment, and learn a bunch of stuff on the way.
The speed of the loading might further be improved by using encoded polylines. Google has a format which renders the polygons faster than the kml-file and there is an algorithm for the conversion. I’ll definitely look into this for speed-ups.
Last step was to assign colors to the polygons based on the values of the json file. I have this working although there is no clear algorithm yet. Through a stroke of luck my feedreader turned up this item today giving me all the information I needed for the coloring, especially through this application. When I have time I’ll use it to further patch things up.
Steps remaining is using the encoded polylines, use jQuery (which should also fix a bug in Firefox of rendering the polygons a bit strange) and give more election results as input. I also stumbled upon Raphael, it looks excellent and might use it for some extra graphs.