If I recall correctly, the current (outgoing) 305-hp Focus RS is officially rated at a combined 30.1 mpg (in the UK). And a recent publication was able to get ~38 mpg out of it in a hypermiling challenge.
Assuming the newer 2.3 Ecoboost engine can pull similar numbers, it does look like there's at least the potential for substantial fuel economy gains over the already impressive 3.7 V6, if it were to become a "mid-range" engine for the mustang.
Of course, drive it like it's meant to be driven, and all that will go out the window. But I'm sure Ford would love to put a "30 mpg combined" EPA rating on the window sticker of the Mustang. It's purely a marketing feature.