Update: Here is what appears to be the Flat Plane Crank variant running through its paces:

The engineers at SVT have been very tight-lipped over the new SVT variation of the S550 Mustang, but today we finally get a look at this car, undisguised – thanks to our friends at Mustang6G.com

It’s interesting to note some key features: prominent hood scoop, functional front fender vents, aggressive front splitter and a front fascia that lends screams the “Cobra” moniker expected of this hi-po model. Also notice the Michelin Pilot Super Sport tires and quad tip exhaust that have been seen in early prototypes.  Also making an appearance is wider read hips and front fenders.  Is Ford taking cues from the Z06 model?

We also see a second Mustang being tested with a much different exhaust note towards the end of the video.  Could this be an SVO, Ecoboost-powered variant?

We are expecting several different models of the next SVT Mustang:

SVT:
GT350 $48k
GT350s $55k
GT350r $68k

The next SVT Mustang is expected to be due in mid-2015.