Certainly the stiffer the system, the better off you are and the higher the speed at which things will go a flutterin'. But I am not so sure that it can be obvious with a twist of the hand that the system is too stiff to get into a torsional flutter mode. Obviously the more torsionally stiff the surface is, the higher the flutter speed and therefore the better off you are, but its still out there somewhere. If you are gonna put weights out in the tip, I'd be real certain to put lots of carbon fiber oriented at 45 degrees in the skin.

Matt, that counterweight is TOO COOL. Was that on the Bandit's (or Stingray???) elevators or ailerons?