I have learned in school that "should" and "could" are polite, although I understand you hesitation to tell someone he "should" do something.
My point?
Not everyone had english as their first language. I always try to remember myself that maybe the other one did not mean to sound so harsh.

His point was not a bad one, by the way.
For example, I have to worry about the performance in a pedestrian simulation of mine all the time, because my fellow student kept using pedestrian models with ~500 polygons.
No one ever sees those details, I told him, a primitive like a cube for the torso are ok, too. In the end I made my own models with 150 polys and the framerate tripled.

(I hope the smileys made clear that I did not want to attack anyone. )