This is gonna suck but "that depends". I believe there have been some w/o reservations that took delivery but that is based on self-reporting and could be prior owners/employees or just messing with people. If it's still 2-4 months and you order now, that'll put the edge of delivery mid December, right before the holidays. So...very little wiggle room. Taking advantage of the tax credit REQUIRES taking delivery not having ordered, not getting a VIN, rather, signing paperwork and driving the car off the lot.
For my wisdom, for whatever that is worth, if you cannot afford the car without $7500 tax credit, you can't afford the car. That said, on Jan 1, 2019 the tax credit is $3575 or there abouts. So it phases out, not disappears. For states with sales/registration tax, a $60,000+ car is going to be expensive every single year. With Congress's new tax code state and local taxes are not deductible. So, there will be high recurring ancillary costs associated with purchase. The SR model will be available in 6-9 months, starting at $35,000 with a ~10% tax credit.