When waiting on my X, I did not get an email several months before delivery. I got it a couple of weeks before delivery, as did many of us who had cars built at the end of Q1 2016. And I didn't even have a P90D, so it definitely came as a big surprise.
If it's anything like previous early builds in Tesla's history, there's going to be a sudden boom in production (end of December deliveries?) followed by a lull, followed by another boom (March 2018), followed by a lull, etc. until they can consistently build them. I'm hopeful it's a bit more metered, but that's not what the history tells us.