The factory 255's are a perfect fit for the rear factory wheels, however the 275's will fit fine on the rears, as stated above, the tire will stick past the wheel a bit, but nothing that will affect the dynamics much.
The fronts could also run a wider size, a 245 will fit great. However when changing the widths of the tires you will also be changing the sidewall size as well. So basically for a 255/35/19 (oem size rear) the sidewall measures 3.5" whereas on a 275/35/19, the sidewall measures 3.8". In the case of the front tires, the factory size is a 225/40/19 the sidewall measures 3.5" and on a 245/40/19 the sidewall measures 3.9".
If you look at the numbers you can see why KIA decided to use those tire sizes, the sidewall measures the same and maintains a leveled ride height.