Actually a mustang magazine did a test on Coil packs, they were able to get 27hp on a stock 09 mustang GT(i believe that was the car)
People still argue with it, but you cant deny the dyno numbers.
I would go with around 65k coils or maybe the 75's.....they get expensive once you get around the 65,000's.
Apparently the 4.6L v8's respond very well to coil packs, alot of people have claimed big gains, while others claim nothing changed.
(guess it depends on the running condition of the car.)
But like the other guy said, GENERALLY the rule of thumb is to use aftermarket coil packs once you've supercharged or are running much bigger numbers.
Although dyno tests have shown otherwise.....
Here's the weblink.