From my point of view...
V6 = promotes the name of Mustang
GT = shows the potential of Mustang
Cobra = shows full potentail of Mustang
Ford GT = What every mustangs wish to be like
Bottom line is, a Mustang is a Mustang and there's nothing wrong with modding them. If you ask why people mod a v6 rather than buying a v8 then they may have many various reasons like stated in previous posts (gas, insurance, etc..)
The whole purpose of asking that question is "why would you mod a slower car when you can mod a faster car?" The truth is.. every car can be as fast as others, people have different goals. Some mod for looks and some mod to go faster.
Stick with what you have and be proud of it. Not everyone has the ability to do what everyone else does.. that's just life. Be safe and happy modding.