The upper arm drop increases negative camber when cornering to increase grip and traction. Its a no-brainer if you don't mind taking a drill to your car.
The coil springs are a good idea to reduce the CG of your car.
Together, the upper arm drop and lower coil springs work very well together. Add a few other misc parts like roller spring perches, adjustable strut rods and you'll have a very good handling Mustang.
Its important to realize, that the shelby "1 inch drop" does not lower the car 1", it moves the upper control arm location by 1". Your net gain is less than 1/4", but the purpose of this is not to lower the center of gravity of the car. Rather its to change the way the suspension moves under cornering. Effectively leaning the top of the wheel in as the weight is transfered to it, thus creating a better contact patch.
This is true, Kingsford is getting a new set of headers after 12+ years.
Seems my auto-x addiction has required the need for headers with more ground clearance as of late.