That's not too bad depending on the make of spring ... so you'd have to be more specific on the make.
That said, you should also consider replacing the struts and dampers (or at the least the struts) now because they'll have them out anyways. The stock struts are more of a weak point than the springs and when you lower them it'll make them worse. Yes, it'll add some cost to the drop but it's either a little pain now or more later since you'll have to pay again to replace the struts if they fail prematurely.
Also consider that you should get an alignment to save your rubber and may need an adjustable panhard bar to realign the back end with that much drop in the rear.