I would have to say that it is in fact proven by the SNELL foundation...
If both helmets are stamped with that seal of approval, I would say that they are both of equal quality as far as crash testing goes.
The Snell tests certify that the acceleration of the headform never exceeds 300 G's. It seems unlikely to me that every helmet tested would provide the same exact results. I'm not trying to be a snob, for all I know KBC's might provide better protection than Arai, but usually you get what you pay for. I just wish Snell would publish the results.
Some things to think about.....nobody seems to argue that Joe Rocket offers comparable protection to Vanson. Why is that? Is it because there's no minimum standard that all gear meets and so the quality of the more expensive product stands out more? Also, I used an example once on the old board: a Volvo (S80 I believe) has the same frontal crash safety rating as a little Civic. Which one would you rather crash in?
I also have some misgivings about the quality control in third world factories (HJC/KBC are Korean-made), but I honestly haven't looked into that enough to make a judgement, it could all be popular myth. My Shoei was made in Japan though