Earlier post implies that you need a cd key minimum to start downloading. So I was wondering are you sure that everyone pre-download regardless if they have registered their cd key? If yes where is this info located?
I know that for Heart of the Swarm's pre-download, even if you didn't pre-order the expansion, the encrypted MPQ's were background downloaded around 2 weeks before release (this was the case since the Blizzard Update Agent which handled this, had no way of distinguishing if one pre-ordered or not). Around 24 hours before launch, the files were decrypted and optimized (so long as the launcher had been run) and a final patch downloaded. (The downside of course applies for those who didn't upgrade to HoTS since that content is still occupying disk space).
Thus if you were entering an actual key, the expansion content would be unlocked with no need to begin downloading the data from that point.
The Battle.net Desktop app (which the 2.0.1 client will require) is a bit smarter in this regard (since it can tell if you've already pre-ordered) but based on Lylirra's post, it looks like all the expansion data will still be downloaded (though locked) regardless.
Basically you are hinting to us that patch 2.01 will drop within this month of Jan if all goes well
I believe this is more in the context of the actual core and expansion content which for the most part, is pretty much complete. That represents the bulk of the data and makes sense for them to begin pushing out earlier to spread out the bandwidth usage. But I highly doubt they will go live with the actual 2.0.1 patch until the auction house shutdown on March 18.