Here is a little quote I read... How Much Drop - Most customers ask me "Is one inch really alot?". When lowering a vehicle, 1 inch is alot. Just think... Add 10 inches to a Honda Civic, and you have a Mercedes Benz S Class sedan. Just 10 inches!! I usually tell my customers to take the drop that the spring says, then double it in their mind... that is how it will look on the Mustang. First thing.... The lower the drop does NOT = better handling. Better handling is a combination of many other suspension components combined with the springs.
I have a 1.5 and my cats scrap on speed bumps with out friends ridging along.. Good springs and Good struts and shocks..