If you had an agreement beforehand, and they changed it at last minute, then you should let Ford know and report it to the FTC. Also remember to write a "nice" google review for the dealer with your experience.
While it's not exact, it's just an estimate as you are right, the drivetrain loss varies due to many other factors. I just went with the basic 15% and did the math, although I wouldn't be upset if the car makes more than the rated 500hp.
In Norcal, I asked my dealer about when they were getting their 2024 mustangs. They told me a few are shipping this week and should be arriving between 9/1-9/7. I'll probably go by next week and see if it's true.