I had a vitz on 15's, and there was NO effect in comfort what so ever specially if you calculate the amount of "volume" you are riding on. i replaced the originally high tyres with wider tyres of a lower profile and according to my calculations i should have had a better ride since more weight was spread over a larger area even though i had slightly more "volume" of air in the new tyres, the problem is finding the correct setup.
I have seen many vitz on large rims and almost all of them drive like taxis with broken suspension, first thing you need ot do is get your dampers checked, once they are found to be in working order then find a set of (light weight) rims which have the following specs:
ET: 38-45 ( i had ET45's on my car)
then get yourself a decent set of tyres with the following specs:
195/50R15 (i had pirelli dragons on my car)
Yes your car will be a fraction slower 0-60 (but as it was my 1000cc auto was quick enough to spank my friends 1300cc manual lancer many times), but the added benefits of better road grip, better braking etc easily outweighs the acceleration deficit. as for those who say that it effects fuel economy, i used to regularly see an average of 15 and better in my car when driven correctly so i don't think fuel economy is THAT bad on 15's.
For further info please take a look at the vitz owners club where i have (many times) written about my experiences with my car and the way it was setup. https://www.pakwheels.com/forumreply_az_TopicID!22811~ForumID!28~pw.html