I can't agree more. Take it from me who has taken a fluid dynamics class. The material from AEM, etc. might be better or slightly thicker, who cares, its effect is minimal. As long as the thing doesn't break into two pieces it shouldn't matter. If the interior diameter of the tube is the same, then they should give you almost the same performance given that you use the same filter. Go with K&N and the generic.

Also, if you think that AEM etc. might be smoother inside, the distance from the filter to the end of the CAI is so short that both brands shouldn't induce be more turbulence than the other.