I agree, the 2 1/2" drop is the killer.
A spring's rate is a function of the number of coils, the diameter of the coils, and the diameter of the wire. Reduce the number of coils and change nothing else and you get a much stiffer spring.
A shock is designed to dampen a spring rate within a certain range. Coupled with too stiff a spring, and the shock is forced to move too quickly, cannot dampen the movement, and quickly overheats just from friction of the oil moving back and forth. What you end up with is blown shocks, uncontrolled bouncing over railroad tracks, and laughter around you every time you go over tracks or navigate a speed bump or dip in the road.
People aren't elitist or conservative when they tell you not to cut springs. The reason people tell you to replace your shocks when you install lowering springs is because the factory shocks just cannot cope with the increased spring rate and you will suffer as a result. Any idea of better handling will quickly be dispelled. And despite popular culture, a Mustang handles quite well for a factory car... Until someone lowers it without installing matching shocks. Then it becomes an understeering bitch with killer oversteer at inappropriate times. Hence "Mustangs handle like crap! I had one with race springs and it couldn't corner to save it's life!"