The comparison of the 6.5X55 vs the 6.5/284 is exactly the same as comparing the 6.5X55 OR 6.5X57mm with the 6.5 Remington Magnum or the 6.5/'06. The .284 Win case has the same capacity as the .30/'06 case, but in a shorter, fatter container. It is prized for its accuracy potential, not its' ability to produce high velocities! There is no reason why the 6.5X55 or the 6.5X57 would not be just as good, except for the need to use a longer action for the latter two rounds.
To take full advantage of the 6.5x284, you will need to use a long action. If used in a short action, the OAL is going to restrict you when using the longer 140+gr bullets. Bullets will need to be seated deeper, thus reducing case capacity.