Some tires go away early in their life. I had some Bridgestones that went out at 20,000 miles, when all other tires on that truck lasted 40,000+. A rwd car is much different than a truck or fwd car. I have driven all sorts of vehicles, but I began with early pony cars. They require a different driving technique, and are not as good on snow and water as a good fwd. However, there are certain tricks that can only be done with a nicely balanced rwd.
I should also mention that asphalt can get especially slippery after a light rain, and any new Mustang has a lot of torque. It requires a light touch, but can be so satisfying when you learn the feel. Also remember that the wide tires will be more inclined to hydroplane. I used to run bias plies with almost the same numbers as the tires you have. Only then, I had less than 200 HP to deal with and half as many gears. Cars are like women, each and every one is different. Respect the difference and you are in for a great time. Treat it like your last car, and you will be miserable.