Less volume, less powder is need to make equal pressure. Same bullet weight and design and same pressure means same velocity. There would not be enough difference between the 284 and 06 for anybody to choose one over the other based on velocity.
In 6.5x284 vs 6.5/06AI I ran into a situation where I was loading more powder into the .284 case than the 06 because of the amount of bullet shank intruding into the 06 case. They ran almost equal velocities, does that mean that the 06 case is more efficient than the .284? No, it only means that they had like pressures.
