You're confusing "fastest" with "best". The Shelby is a monster, but the S550 is by far the better all around car. The only thing it lacks is power, for now. It has the better chassis, suspension, engine, interior, etc. Really, the only thing that I think I'd swap from the Shelby to the GT is the TR6060.
I sure am not.
13-14 Shelby is the best Mustang produced to date. We dont buy mustangs for a "pretty" interior or for ride quality, we leave that to the soccer moms. Want nice interior, go buy German. Want ride quality, go buy a cadillac.
Fact is that the shelby destroys the s550 in every performance category, where it counts.