One time I sat down and did the calculation for how much energy was tied up in the angular velocity of a bullet compared to the linear velocity component. To facilitate things I used an idealized shape whose angular moment of inertia wasn't too hard to calculate, and the shape would actually overestimate the spin component of the energy compared to that of a regular bullet.
Nevertheless, the angular velocity component was a quite small percentage of the kinetic energy, and the difference because of different twists was not nearly enough to explain the velocity claims of slow twist proponents.
