Lowered my GTi with Boge -35mm springs. Rears are dead easy to do and should take you no more than half an hour. The fronts are a little more difficult. You'll need a pair of coil spring compressors to enable you to take the top mounts off safely. All you do is undo the top nut on the strut mount, Undo the Anti Roll Bar, take the two bolt out that hold the strut into the hub. Knock the hub out of the strut, Remove the strut, slacken the nut that holds the top mount on, Apply the spring compressors, wind down till spring starts to move, undo top mount, remove spring. The fittment of the new springs is a reversal of the above.
You can whilst you're at it, also fit new shocks (Uprated) but the standard ones are ok if not showing signs of wear/leaking.