You need to look a little bit at the history of Tesla selling cars. Let's just optimistically assume that they actually deliver vehicles Q4 2017 (they haven't hit an initial delivery date yet). Let's say they do though, it won't be all cars with X options shipping by the end of the year, it will likely be a handful of cars delivered on stage New Year's Eve 2017 at the Model 3 Reveal Event (very similar to the Model X reveal). There will be no signature cars, so instead the highest battery pack performance model will ship first. You can expect this will cost at least double the base price of the Model 3. For Model X the first 6 months of production vehicles is looking to be performance models only. Given the interest in Model 3 and the desire to get the full tax break, it is certainly possible that they only delivery performance models for the entire first year of production. If you were wanting a base model, I wouldn't count on it before 2019, and I wouldn't count on getting any tax credit. This is why Elon is quoting the price WITHOUT tax credits.
Sigh, you might be right about the performance versions. I don't really want a P and might not be able to afford to get one depending on the cost differential. Right now the P upgrade is $20k. To go from a base BMW 3-series to a M3, its over $30k. That doesn't bode well for me keeping the car in the mid-50's and still being able to have all the creature comforts.