I'd go 18's for ride quality and the ability to run on a track with 14" rotors and ve brembo's on my vy. I currently have 19" rims, but harsh but im lowered on springs as well. 20's is just idiotic and purely for the look. And yes 18's would provide you with a quicker car if you were set on a certain brand/model rim, the larger the size the more rolling weight you're adding to the car and essentially yes it will be slower. 20" would be slower than a 19 or 18, probably won't be noticeable though.