Actually, I believe heated seats come with the Comfort Package. I don't know if this package is available without first getting the Premium Package, though.
Yes, heated seats do add weight. But it's also really nice. As a bonus -- you get the pony lights.
2013 Mustang V6 w/Premium Package, Comfort Package (Yay pony lights...), Appearance
Mods: Airaid CAI, Ford Racing 3.73 Gears, Aluminum 1-pc driveshaft, American Muscle Staggered Black Bullitt 18" wheels, Bama Tunes, Ford Performance engine cover, JLT Air/Oil Separator, Boss 302 Strut Tower Brace