I wasn't going to post but I don't think the OP frequents this forum anymore as he is now on the Lotus boards
I'm the current owner of this car.
As far as I know, nothing had to be done in order to fit 305's in the rear. I'm not sure if there are performance differences between the 19's and 20's. I've only driven the car on the ADV's. I'll be putting the 19's back on fairly soon though so I can report back after doing so.
If you'd like to see some more pics of this car and the ADV7.1's:
http://www.m3post.com/forums/showthr...hlight=content
I'm actually thinking about selling these sometime soon. Keep a lookout for this set in the Classifieds.