Like Lou says mass and aero are totally separate. That big lump of lead on the P38 elevator is striclty a mass balance to help avoid flutter. It did nothing to aid the pilot with moving the control.

I don't think this is true. Certainly a flexible wing can aid the formation of flutter but it's not a requirement in itself. Flutter of control surfaces occurs due to an OSCILLATING energy acting on the surface at a frequency that is the same as the resonant frequency of the control system. If the energy input is not cyclic, as in providing an oscillating force, then the system will not be forced into resonance and the surface will not flutter. Think of a violin string or the blade of grass between your thumbs. Part of the resonance equation is fles in the system through either structural flex (my soft aileron wood in that 1/2A model) or slop in the system so that things can flop around. If the control and activation system was TRULY rigid then flutter could never occur. But all practical construction and systems can flex and have slop inherent in them. These are enough to cause flutter. Consider that strip ailerons are much more prone to flutter than conventional designs are. This is because the surface is so long and slender with the control input at the inside end so the surface is free to twist in the wind and can often flutter. The energy to cause that comes from the airflow which would indicate that there's a "noise" generator at work on the wings that has a cyclic effect on the air flowing over the wing. This "noise" changes frequency with the speed and if the frequency reaches the resonace frequency of the control surface system it's flutter time. Adding weights can raise that frequency by moving the balance point closer to the leading edge of the surface. If it moves the resonance point to where the model cannot fly fast enough to generate the noise frequency to match then there's no flutter any more.

Howwzat?