FWIW I agree with your theory, Roger. Any restriction to air flow acts just like a conventional flap choke (on a fixed venturi carb). It increases the depression 'seen' by the fuel jet, and hence the fuel flow for a given air flow (meaning a richer mixture). One way to compensate would be to vent the float bowl to the inside of the filter (which is exactly what the later 'emissions' carbs do, and partly why they do it).
But my paper filters don't seem to present enough restriction at idle to make a noticeable difference. I get the same results with them on or off. If your filters are offering a significant amount of resistance at idle, then what are they costing you in full throttle, redline power (where the airflow may be 100 times greater) ?
I did some experiments once with the O2 sensor from a newer car mounted in my TR3A exhaust manifold. The results were somewhat surprising, in that the meter would swing from one extreme to the other with the slightest change in throttle. Trying to tune the carbs to be mid-scale (theoretically just enough air to burn the amount of fuel completely) under cruise conditions was both frustrating, and counter-productive. The car ran better with the "lift the piston" method.
But I still hope to pick up a "wide band" exhaust analyzer some day, and see what I can figure out with that. With gas projected to be $5/gallon by summer, it could easily pay for itself in just a few years.