Everything we've heard semi-officially is that the Model Y has been designed in lock-step closely behind the Model 3, to maximize common parts and manufacturing. Very likely a good chunk of the engineering talent moved over to work on it after "Pencils Down" on Model 3 last Summer. While the record reservations of the Model 3 may have changed the timetable a bit, I just can't imagine that Tesla's plan will be to hault all expansion of their lineup until Model 3 reaches a production equilibrium. There's going to be a glut of orders by the end of this year, but you don't plan your production capacity based on initial launch demand, but on longer term consistent demand for a product. So with Tesla's production capacity plan to get to 500,000/yr during 2018 and then 1,000,000/yr sometime in 2020, plus whatever capacity is added by the European Gigafactory 2-- It makes sense to bleed in Model Y production as they expand, I'm betting sometime in 2019 as Fremont part 2 expansion starts opening up.