It shouldn't be too bad. We have been downloading a lot of the HoW files in the last few updates. All we have left to download are a few packages and scripts that allow us to access the HoW data, and a few scripts that actually make sense of the data already downloaded. In other words, you already have most of HoW downloaded. There's still all the server-side files that have to activate and the final download to activate your client-side files, but it should be a very small download. That way you can play the DLC within minutes of it dropping, even though everyone will be downloading it at once.

It'll be a small hot fix. We already downloaded it. They just need to unlock the content :)