I DD my 2013 GT.
The all-seasons were tolerable in some pretty bad road conditions this year but I'm planning on getting some winter tires next year. The Brembo and Track Pack cars come with summer tires (as do the V6 performance pack cars) so you will *definitely* want snows if you go that route but it wouldn't be a bad idea anyway. Even on dry roads the A/S tires can get skittish when it's really cold out.
2013 GT Premium 6MT. GHIG, Black/Cashmere 401A interior, Comfort Package, 19" 5-spoke wheels. Two boosters, one RFT card for my parking garage at work, daily driver.