I don't have a problem with the 20 weeks as such (though for me its now looking like 24 at the earliest). Last car I bought was about 10 weeks. They estimated 8 (max 12) when I ordered and it got pushed back 2 from the estimated 8.
What I have an issue with is:
a) Estimates are hugely out. Not just 1 or 2 weeks... its like an extra 3-4months!
b) VW have some immensely dumb allocation system where each dealer seems to have a "hidden" allocation so when you order you're basically flipping a coin on if you'll be taking delivery soon or never. And how do you find out which it is? When the car arrives, that's how.

c) They still can't offer any explanation.
All of the above are very very easy to fix. .... If they cared that is.