If you would be satisfied with the stance of Eibach proline springs...why the interest in coils?
Eibach sportlines are their lowest drop rate springs.
You'd be looking at also adding dampers made for lowered cars if you drop it that low. Though, not with the proline springs. Teins are also a drop that would require suited dampers. You can continue to run your oem dampers until they begin to leak, and they will eventually, but you may want to consider reducing the cost by installing the suited dampers at the same time you do the springs...as you will be there anyhow. If you have corrosion issues...cheap coils with lesser quality and warranty would be throwing money away.
You can get performance from both springs/dampers and coils as the gti really isn't a true coilover in the rear. It is adjustable, but does not relocate the spring out to the damper location. So you really need to decide for yourself if you would be satisfied with the stance of springs, or if you want to go lower with coils. Then all that is left to decide on is quality.
I have H&R supersport springs, Bilstein B8 dampers, but I have spring adjusters and coilover springs in the back....sort of a Frankenspension. I have run Eibach sprotlines with oem dampers, H&R supersports with both oem and B8 dampers, and the current Frankenspension. Eibach sportlines were the most comfortable, but the current set-up outperforms all of the previous ones I had on the car to date.
