Just finished up selling my old car and I started my hunt for the perfect GT. I have access to a X Plan PIN and was curious what I should be paying for a 2007 GT Deluxe with leather or a Base premium for out the door, Both 5 speeds.
You can get leather as an option on the Deluxe (listed at $695 on Ford's site) or the Premium has leather standard as well as the Shaker 500. If you want leather and the Shaker, it's a couple hundred dollars cheaper to get the Premium with them standard, rather than add them as options to the Deluxe
You may want to try posting under "From the Ford Salesmen" under forums on brad *******'s site. There are quite a few salesmen over there who could probably answer your question the best and you may even be able to find one close enough to do your deal.
EDIT: I'm screwing up posts left and right tonight, didn't know we can't post other mustang sites...pm me your e-mail or click the little aol guy under my post count and I'll give you the address
With the X-plan you still get any incentives or Interest rates taken off the discounted price. This is the only way I can see you getting invoice or even lower.