Yah the stock leather seats look alright, but really don't have much support and aren't very comfortable after a long drive if they're pretty worn in.
Fortunately for you, you have black leather/interior. You can probably find Cobra/Mach 1/Bullit seats out there...from either a wrecked car or a private seller whose upgraded to racing seats.
There are some fairly mild Corbeaus for a reasonable price...these in black wouldn't look too bad.
MUSTANG CORBEAU SEAT RECLINING GTS II PAIR
12 Black GT500 SVT PP l VMP TVS l JLT123 l O/R X l 28x11.50Rx15 M/T Street R's l BMR Suspension l Recaros
03 Sonic Blue 3.8 / 17 Magnetic Fusion Sport 2.7 EB