I sell springs by all the big brands and was thinking of Eibach Pro kit £185 or an Apex kit £130 although both will lower by 25-30mm, the only way you will get 15mm is to buy a set of GT springs (Although as the GT chassis also gets different shocks I do wonder how much of the 15mm it will give you? I know with the passat fitting the springs only on std shocks give less than springs and sport shocks together)
(I have looked to see if any springs are available after market but no body lists them as yet!)