ESP was the biggest 2001 change, 2001.5 brought the new turbos/oil lines, new tranny/shifter, new suspension perches, revised engine piping (no F-hose), etc etc etc....

Some older 2001 cars seem to have the newer turbo part numbers, but from what I've seen (and heard from people who have had them pulled), most pre 11/00 don't. It seems all post that, do. (Ie, new badge cars).

Tires weren't anything that "officially" changed, its just luck which you got.

Boston Audi Group Website