The higher the price goes, the more a 'somewhat better' mustang isn't going to cut it IMO. What I mean by this, is the penalty paid for being based on the same platform as the basic V6 mustang.
As the price extends beyond 40K, the more people start thinking about other alternatives, vettes, porsche, BMW, etc and so forth. One begins to ask the question, best new mustang or slightly used 911? If the price for these mustangs is going to be 40, 50, even pushing 60K, why get a car that is compromised in whatever for due to the needs of the V6 base car?
Such a thing would work for the niche that wants such a mustang. But there is competition out there performance wise in the US market.