15mm wouldn't be noticeable.
I've lowered mine 40mm on the JOM shocks and springs kit from dpmperformance and it's a really good kit.
The front only looks a bit lower but the back seems to have dropped more.
I wouldn't bother with the time/money put into only getting it to gti height, if you ever do put say, 18" alloys on it'll look like mine did (my dads land rover!)
But, it's up to you! If you don't want it dropped too much, at least go for the Eibach 30mm set.
Good luck
