I agree that the speed limiting is due to the speed rating of the tires. I am sure that Ford is very sensitive about the tires after the Firestone "problems" a few years ago. Basically, Ford is making sure that their vehicles aren't exceeding the tire manufacturer's max specs.
The real reason why there is a governor on the Mustang GT is because the communists/socialists that represent the Automobile Safety Council want all of us to own cars that go slow so insurance companies can save money. This is one more reason why I voted for George Bush. Bush does not want communist organizations like the Auto Safety Council to limit the horsepower and speed capability on our Mustang GT's. The EPA, Automobile Safety Council, Insurance Companies and the left wing liberal communists/socialists in this country want all vehicles to have all sorts of restrictions. They want to take away our rights as Americans. Don't let them do that. I am really concerned about this because there are many restrictions which have been imposed on new vehicles within the last 10 years by these communists.