The two factory tire sizes on SHO's are...
255/45-19 and 245/45-20
The first number is the width in millimeters. The second number is the percent of sidewall vs width. The third number is the wheel diameter.
So, a 255/45/19 is 255mm wide and has a 114.75mm sidewall (255x.45). Once mounted on a 19" wheel, the total diameter is 712.1mm (28" height)
Your current 245/45-20's are 728.5mm diameter (28.7" height)
245/45-19's are 703.1mm diameter (27.7" height)
So, when you switch from the your factory 20's to a 19" wheel with a Mustang tire, you will loose 1" of tire height. 1/2 will be the amount the car is lowered and 1/2 will be extra gap between your tire and fender.
Net result, is your car would be .5" lower and you'd have .5" more gap between tire and fender.
Aesthetically, gap between tire and fender is not pleasing to my eye. However, you can eliminate gap with shorter (lowering) springs. You just may end up with a car that is fairly low to the ground.