I agree with your methodology, it is always better to think about the decisions and not just go with what others tell you. I'm just trying to help you get to the truth by sharing my own thought process.
I think everyone will attest that with a forced induction system there is much more flow through the intake when in boost. You are correct, the engine displacement doesn't change. But, how do we increase the density of the air in the manifold, except by drawing more air from somewhere else and cramming it in? That air comes from the intake. The s/c pulls a fixed volume of air from atmospheric pressure, and rejects it to the manifold at manifold pressure. If the manifold pressure is any higher than the vacuum pressure, the car is pulling more air through the intake than a N/A car would. To relate it back to what you were saying, to conserve mass, the volume flow rate of the less-dense air in the intake must be greater than the volume flow rate of the denser air in the engine.
But you are right, the point is moot if you have a N/A car. The volume flow through the intake will always be less than RPM*Displacement/2.
EDIT: To clarify, I am just trying to explain why the upper limit of 451CFM the guy from K&N quoted does not apply to a FI car. His method of computing that number only applies to a N/A car. And thus so does his reasoning behind filter selection.
