Is it possible that his mustang has been lowered already? I've never heard of varying ride height between new edge mustang year models. The 18 inch wheels you have should help fill that gap, but it also depends on what kind of tire you're running. My v6 mustang had 225/55/16's, but I bought new 225/60/16 tires due to an availability issue. It made a noticeable wheel gap and ride height change (had the 225/55/16 for a year or more).
Either way, dropping your ride 1.5 inches will definitely give your car a more aggressive stance. Also how wide are your tires? Staggering the rear wheels (with spacers or wide wheels) gives it a much more planted look.