I can honestly say that my 1987 Mustang LX coupe 5.0/5-speed with nothing but a K&N filter, and the mufflers removed (replaced with straight pipe) - but kept the 4 factory cats. *the exhaust still exits out the back and looks stock.
the car has 70K original miles. 13.8 with street tires (16" wheels)
13.1 with slicks (15" wheels)
I have found that the coupes i have driven haul ass compared to the GT's, hatchbacks and so on (all of the coupes are 5.0 5-spds that i've driven)