I've been running a set of 19's on my mk6's for two and a half years. There is little difference in ride quality between those and the 18s (I ran several different sets of 18" wheels on my first mk6 so I do have plenty to go on). There is a difference but not a huge one. My 19's are wider and running a different offset to the factory standard too.
As the mk7 is supposedly better riding than a mk6 (I've not had the opportunity to drive a mk7 GTI yet) I would think you will have no problems at all.
In fact I'm planning on probably using the 19's off my mk6 on my mk7 too once I've put some new tyres on them.
![cool :cool:](https://www.golfgtiforum.co.uk/Smileys/modern/cool.gif.pagespeed.ce.P1E-cP-Vxt.gif)