Is there a difference? It seems to me the main difference between Gt and V6 suspension is a thicker front sway bar and an additional rear sway bar on the GT. The V6 weighs 150lbs less than a GT so if anything (if they're not exactly the same springs), the V6 would sit higher - but I reckon they're probably the same as it would make the unit cost cheaper.
I've got Roush springs on mine and they'r comfortable and lower the car just right in my view and you can still get the car on ramps.
The front spring rates are different but the rears are the same. (coupes) If your looking to drop the rear even with the front, a set of Roush rear springs will do the trick. They bring the rear to within 1/4 " of the front. If your looking to lower then you'll have to go Eibach, Voightland, Tein, Tokico etc.