My car has also been lowered with Eibach pro-kit springs, which are unfortunately not available anymore.
I used to have 18 inch wheels:
I did not notice much of a change when I went to the 20 inch wheels, except of course if you hit a pot hole or something, then wham, just not much rubber to absorb the impact. But highway crusing is very smooth.
The key is make sure you get hub centric wheels and not lug centric wheels, that way there will be less vibration from the wheel itself introduced to the ride of the car.