Actually, your answer is only partially true....yea wider tires give you a bigger contact patch with the road surface, but they only increase the handling of a car up to a certain width specifically for each car.
For example....Im a part of the Formula SAE program at Alabama, and we are trying to get the narrowest tire possible for our races.....simply because it is what works best for our competition.
For the Mustang however, I understand there is a substantial amount of more power compared to our car powered by a Honda F4i engine, but besides that you must look at what you are going to be doing with the tire...road racing, drag racing, etc.....What Im trying to say is there is a limit to where the optimum tire width should be for a particular setup of a car(horsepower, type of race, vehicle weight). I dont want to line up at the strip with a 500 hp mustang with 6" wide tires, but I also dont want to line up with a 20" wide tire either(a whole lot of rotational mass which would slow track times, waste of money and just plain stupid). All in all like I stated there is a limit to each setup of what is best for a particular car. I used to run 12.5" wide tires on my jeep, now I run 9.5" wide and have A LOT more traction in the mud......so WIDER DOESNT ALWAYS GIVE THE BEST TRACTION