They will definitely give expected ship dates for the various configurations before people make their orders (they certainly did for both Model S and Model X).
I don't recall that but perhaps my memory is fuzzy. I recall there being a lot of teeth-gnashing as people with low reservation numbers but configured 60 packs or coil suspension or non-P or whatever simply being passed over while folks with higher numbers but choosing more expensive options were given their cars. I recall the users here on TMC figuring out that they were batching cars. I certainly was not given a "ship date" until 3 days before the car was ready. Note that I have not been involved with Model X in any way, shape, or form so maybe they've improved their communication.
Further, supply chain problems (like the grey interior issue noted above) are unknown at the beginning but could delay production.
The bottom line is that you are dealing with a company that completely sucks at communication and is still making up nearly everything as they go along. Every product they've ever shipped has been late, the misinformation within the company is legendary (different SC's telling different stories that also contradict what's being said by corporate folks, etc). Here's the law on the tax credit:
From:
Plug-In Electric Drive Vehicle Credit (IRC 30D)
Qualified Plug-In Electric Drive Motor Vehicle Credit (IRC 30D) Phase Out
The qualified plug-in electric drive motor vehicle credit phases out for a manufacturer’s vehicles over the one-year period beginning with the second calendar quarter after the calendar quarter in which at least 200,000 qualifying vehicles manufactured by that manufacturer have been sold for use in the United States (determined on a cumulative basis for sales after December 31, 2009) (“phase-out period”). Qualifying vehicles manufactured by that manufacturer are eligible for 50 percent of the credit if acquired in the first two quarters of the phase-out period and 25 percent of the credit if acquired in the third or fourth quarter of the phase-out period. Vehicles manufactured by that manufacturer are not eligible for a credit if acquired after the phase-out period.
So my guess is that if you reserve early you'll get at least 50% of the credit regardless of your config. If you're not planning to get a loaded car I would budget based on that. Also, as others have said, allocate your budget to hard parts (battery, motors, tech package, etc) vs software (autopilot, supercharging) as the software things can be enabled more cheaply later.