You probably wouldn't notice 2.5 mm if it was an increase in the length of your nose.
Seriously, it's a nit. If you were trying to cram the largest wheel you could fit in there you might start worrying about a couple of mm.
Since the Mazda rim is 1/2 inch narrower than the Kia, if it had exactly the same offset as the Kia it would be 1/4 inch narrower on each side. Since the offset is different by less than 1/4 inch the Mazda wheel will still stick out slightly less than the Kia wheel.