What's a good amount of Durametric lowering that keeps it comfortable?
 

Really impressed with the Durametric lowering!

Did just 25mm all corners for my non-GTS model with 21" winter tires.

I must admit the ride suffered a bit more than I thought of here in crappy roads. So, does it make any difference if I raise it up a bit? How much?

I have race cars with low suspension and this is my winter daily so more comfort could be appreciated...but going back to stock just is not an option for a car freak :D

You just have to play with tire pressure and height. There are not only different ride preferences, but also different ride height/ tire pressure combinations. Everything you change will be a trade off somewhere else. Try different tire pressures at different ride heights, keep a record, and select the combination that works for your expected use. You will likely have different combinations when use changes- in town, highway trips, off road, snow, sunny dry..... The experimentation is part of the fun.

Once you find the right combination, make sure to get an alignment with the suspension set where you will drive the car most often. If you don't, then you'll go through tires very quickly.