Some here have mentioned gas mileage/economy - the OP even commented on the "worse than expected economy" of the twins.
That hasn't been my experience - stock, I was averaging around 34.5 mpg in exclusively highway driving.
And then this happened last night on a 25 mile drive at an average of 62 mph:
Yes, I realize the Average MPG gauge/meter is not precise, but that's pretty freaking amazing.
Of course, I did a couple recent mods, so I'm not very close to stock anymore...
BTW, I do appreciate the write up, OP. I'm interested in Mustangs - especially the new '15, but the fact that every 5th car around here is a Mustang has always kept me from seriously considering one.