I bought my car with basic audio, spent my money on other goodies. I was planning to swop the speaker anyway, as usual the factory speakers are just some plastic thingy. I looked at the speakers, and here is some basic info on them:

Front tweeter: 8ohm, 38mm dia and decent magnet size. It has a 10.7μF capacitor as crossover, 6db/1.9kHz crossover frequency.
Front mid/bass: 4ohm, 8" size
Rear tweeter: I have no info
Rear mid/bass: 4ohm, 6.5" size

After looking at the front tweeter, I decided that I probably live with it. It was big and looked nice. On otherhand the two mid/bass speakers were just plastic construction with small magnets, thus they got swopped.

I bought used speakers from ebay, took them apart and glued the new speakers (Audison) in it with silicon based glue and added some foam surround to it. After taking the door card down, it is easy two swop the speakers. See pictures of the original speakers, the plastic body and the finished items below:

Front speakers:
Audi original front.jpg
Audi front cover.jpg
Audi front replace.jpg

Rear speakers:
Audi original rear.jpg
Audi rear cover.jpg