Most drop around .3-.4 tenths. Supercharged cars and serious drag cars usually see .5+ off their 1/4 mile times. Not saying a lightly modified car can't reduce their 1/4 mile time by .5 tenths, however, its not something that happens all the time. Its basically going to depend on what else is done to the car and how high of a stall you get.
If ANY of you are thinking of getting a aftermarket torque converter, I will let you know now that anything shy of a 3,000 stall is a waste in my honest opinion. That is what I have now and I feel there is still room for plenty more stall. I'm letting you know this now so if you decide to sink the 700+ into a torque converter, you get the job done right the first time.
One last note. EVER automatic Mustang NEEDS an aftermarket transmission cooler. It is a MUST HAVE modification. Heat is the number 1 killer of automatic transmissions and these 4r70w automatics run hot stock. With aftermarket mods, like gears and a torque converter, they run even hotter.