So I previousely had 205/55/15s on my car but since I have rear hub spacers they are just a tad too wide and occasionally scrape. Also I noticed soon as I went to bigger tires I could only describe the steering as more "heavy" and requiring little more effort on turns almost as if the car was a little less agile.
I just swapped them out for some hankook ventus in a 195/50/15 which just feels like the perfect tire size. I still get a bump in performance over the stock width without going too wide. For a small as a car as the yaris seems to be plenty. Also with the smaller sidewall and the fact that I can jump to a performance tire now with a stiffer sidewall in this size range i dont really loose much traction at all but gain a lot more handling. The car is now quick in turns to react. It feels like there is a bit more pickup which is most likely due to less unsprung weight and rotational mass as well as a smaller overall diameter. Only downside it takes slight getting used to my car again as where I used to have clearance over bumps now i scrape my under bracing and occasionally scrape my front bumper now on the driveway as the car sits a bit lower now. However with the smaller sidewall it does give the car a bit more of a aggresive look.