Apparently, there are plans in the works for the 2015 Mustang to receive a 4 cylinder engine, allegedly making the same power as the current v6, which all but ensures that no v6 will be offered.

Obviously nothing's final, but the main contender seems to be a 2.3L EcoBoost (meaning turbo-charged 4) making a peak 310hp while still getting 34 mpg on the highway.

Personally, I'm thinking the numbers that Ford have been putting out for years now are more engineering based than real world - I've driven a lot of 300hp cars (own 3) and the v6 '305hp' Mustangs I've driven feel more effectively around the 240-250hp mark.  I don't doubt that 305hp is the peak number they got on the dyno somewhere, but when it comes to usable hp, it's all about torque and the area under the curve. 

I'm not expecting the 4 cylinder to improve things any.  Although it might be one of the best looking economy cars out there. :D