Red, my old LX ran mid 13s with just gears and exhaust and a couple of bolt-ons, and I've recently driven a completely stock (down to the tires) '93 Cobra to a 13.7@101. The first '99 GT I saw at the track (with the paper tag in the back) ran 13.80s and I have a friend who has driven a stock (he removed the air silencer from the filter box) '00 GT convertible with an automatic to a 13.97 - two other board members, Spanky and Sensei, can both verify that.

Something is wrong with your wife's cousin's car (though you didn't mention heads in your list of mods and the stock E7s are barely fit to be boat anchors). Heads, cam, exhaust, intake (Basically the airflow part of driveline) on a stock short block, manual tranny 5.0 makes a LOW 12 second car with a proper driver. Mine ran 12.0@111 with 4.55 gears and slicks with ported iron heads, E-cam, Mac long tubes, Flowmasters, Edelbrock intake, and a few other bolt-ons. Only tuning was done with a timing light and an adjustable FPR.

I know at least a dozen non-power-adder stock short block 5.0 Mustangs running in the low 12s/high 11s in smog legal form.

A set of Twisted Wedge or AFR heads and a E-cam on Millhouse's car will be worth 70+ hp to the wheels.