Nick, i agree that not much energy is 'lost' by the balancer shafts through energy conversion to Sound/Heat etc. However the KPE (The Kinetic Potential Energy for the benefit of others!) acts like a flywheel, and would only ever be redeemed upon engine coastdown.
Therefore, when applying full power from the engine, lets say 10% gets sapped by the WP and Balancers spinning them up. They will only give that energy back when you DONT need it (off the Acc pedal).
So if you were driving economically, indeed not much energy defference would be noted by removing them, because you are using the engines rotational inertia to help manage your speed. On a rolling road or drag strip however, you never get the chance to use this inertia, not even on coastdown inertial dynos, as these are usually done in neutral.
