I could see the Mustang getting special treatment just because, well, Mustang.
You're definitely right about Ford's pricing and engine arrangement on other vehicles. Even the Fiesta is included, the more efficient and powerful 1.0L is an upcharge.
I just think that it's going to be hard to sell most Mustang buyers on efficiency and not HP. Beside the fact that it's a *gasp* 4 cylinder, it's going to cost more than the 6. Doesn't bother me personally, since I'm pretty interested in the new Mustang and the 2.3 is the one if get. Lightness, torque, and dollar-per-hp on mods have me intrigued. So I'm pulling for at least 310 hp stock, but it wouldn't bother me if it were less.
Another thing to consider is that the 2.3 will be the biggest seller, and possibly only engine choice, in some European markets. It ought to have some decent HP to make both sides of the pond avoid feeling like they're getting shafted. We know it will be a capable engine since it will be used in the ought-to-be-300-hp Focus RS.
Of course, all speculation until at least Dec 5, when the 2015 will be unveiled worldwide.