Dealer markup on a Mustang?
They are going to be a dine a dozen in a few months. No way in hell I would pay more than MSRP, and to be honest I wouldn't even do that. Just hold off a couple months.
1st year car? With a mark up? NO thanks. Let someone else pay extra to beta test it.
You already have a bad ass, and pretty much new, Mustang. Enjoy it!