Handling is improved to be honest as the tyre width is wider e.g. better grip! So its all gains and no disavantages really in my eyes, the only down side which unless your an old man and likes a smooth ride on a fast hatch is the vibration thou the tyres is a little increased due to the small profile on the tyres, but its not massive difference just noticeable on the not so perfect roads.
I've changed to a set of 19"s that allow better clearance on the brakes, got a set of Porka GT3 calipers going on it, and the old ones wasnt very good for clearing them, where the new ones are alot better, once i got the camera sorted I'll take some pics on the new wheels to post up in the forum!