These K&N debates are always fun…

DANSMB:
Easy answer, your stang either had enough air already, or it wasn't tuned to take advantage of the new filter. A stock engine probably isn't going to see much if any bennies. I like it because it's reusable and doesn't get restrictive as it collects dirt. But on my chevota I can tell blindfolded if someone were to switch in a paper filter, the benz motor I cannot.

AF300E:
Sorry, the motors air/fuel system are not factory tuned for performance like we hot-rodders call performance. Unless you meant tuned to the lowest production costs and California emission standards? If they really only had performance and filtering in mind, then most all filters would be much larger. The factory is a business, which filter/airbox combo is cheaper to produce? Every single part goes thru the bean counters, if they don't pick the cheapest one that’s “good enough” today, it's because they believe it won't be cheaper down the road. Eg; recalls, emissions, politics, tarnished image, etc.

I’m really surprised you chose to attack pipes (you said dirt so I'm assuming 2-stroke).
There is no single mod I’m aware of other than nitrous and forced induction that makes such a HUGE gain in performance. And they’re not a razor thin band either. Most people I know don't even bother to ride their bike until they buy a pipe, and quite often it's negotiated into the original purchase.
I have no clue why the bike mfg's do not put better pipes on their bikes. Maybe it’s “power fixing”, remember when the Japs got busted for price fixing???