Haven't been an EXi owner, but we had a VTi and a City. VTi had it's original wheels and tires, but the City had been fitted with 16 inch rims, and new dunlop tires (can't remember the width though)
From my experience in riding an EXi with wider tires, you have to be careful in choosing the right brand and make sure the tires are new. I was in an Exi on the motorway with after market tyres that were unbearably loud; it was on standard rims, though.
Having bigger rims (only 15's) wouldn't really affect your ride quality in my opinion. If you had a car like the City, and you put on humongous rims on it, then it'd be affected quite a bit. We had that experience with the 16's on the City.
Having wider tires would give you more grip too, which would be especially helpful during raining season (which is coming up!) and during everyday driving, of course.