it's not harsh in any way
i'd say about 20% less comfortable if that makes sense but worth it for the looks and improvement in handling
my wheels are 18s
eibach state a 50mm reduction for the sportlines but the springs are for TDis and GTis
the GTi is about 15mm lower than the TDi so brings the GTi down about 35mm
i changed them with my bro in law who is a mechanic in his workshop on a 2 post lift
the back is 10 minutes a side, very easy
the front can be done a couple of ways, if 1 person doing it you would need to take off the driveshafts and hubs to then be able to hammer off the struts from the clamps
we did it by levering down the wishbone while the other person hammered off the struts so not needing to take off the drive shafts
he has hydraulic spring compresors, much easier and quicker
took about 2.5 to 3 hours all in
but i wouldn't recommend trying it yourself unless you have the right tools and someone to bail you out if you get in trouble
hope this helps