So an alternative perspective: I had an early (first 2 weeks) build S550 2015 GT. It spent 2 month in a QA lot to catch issues before shipping / fix and still had a bad dashboard full of rattles and terrible panel gap issues. It landed with 30 miles on the Odometer with Dealership PDI. I don't...
I have the 10R80AT on my 21 GT PP1. When I first got it, the break in / learning period was awful. First 1000 miles you thought the thing was broken, lots of hunting for gears, hard shifts, etc. After it got out of the break in period, mostly smooth usage aside from 3/4 sometimes being a hard...
I had a 15 GT and have a 21 GT right now. The 15 had panel gaps all misaligned because it was built a month into the run of S550s. It also had massive problems with the interior rattling around the dashboard and glovebox. My 21 has neither of those problems. Out of them guess which one I cared...
From what I've seen its not part of the plan to deny on a per car basis but only to deny taking the whole plan. So they can't just cherry pick and say nope, not for the Mustang. If they are trying to pull this with you, find a new dealer.
I asked my dealer about Xplan prices and had already mentioned I was a Costco member. According to the dealer the Costco discount is better than XPlan, so if you are a Costco Member, consider asking about that.