I don't have the source, but Elon said something to the effect of well-equipped cars will get built first. And this makes sense for any car, but especially the Model 3 where the profit margins on the base car are expected to be not very large, so the company will make a great deal of profit from the add-ons. So if you're in charge of making money for the company, it makes sense to roll out cars that make you $10k in profit vs those that make $5k in profit.
I still think there's going to be a correlation - no matter how weak - between reservation numbers and time of delivery, so I'm reserving on the 31st even if I don't spec it all the way out. In fact, because I'm not going to max it out (likely), that encourages me even more to show on up 3/31, as I really want that Federal tax credit, and if I'm not getting a Model 3 until halfway through 2018, I want to be in that first batch of <$45,000 buyers.