Extra velocity does not follow extra capacity. Instead it is 1/4 of case capacity with both cases loaded to the same pressure.
If the increase is 5.5% of powder capacity, then the increase in velocit would be 1.375%. If we assume the regular .30-06 can get 2800 with a 180 (and it can, easily, from a a 24" barrel) then the Gibbs would be capable of a gain of 38.5 fps-again, at the same pressure.
