Here we go again.
When handloaded (and most people who post here will handload) the 6.5x284 gets AT MOST 150 fps more than the 6.5x55 with 140 grain bullets AT THE SAME PRESSURE.
This is due to the 1/4 Rule of internal ballistics, which states that any increase in powder capacity results in only 1/4 of that amount of increase in muzzle velocity.
The 6.5/.284 Norma holds about 60 grains of powder with a 120-140-grain bullet loaded. The 6.5x55 holds around 50 grains. This 20% difference amount to a 5% difference in potential muzzle velocity.
The 6.5-284 is capable of right around 2900 fps in a 24" barrel with a 140-grain bullet, while the 6.5x55 can get around 2780-2790. This is with published, pressure-tested data for both cartridges.
Additionally, the 6.5x55 case will normally feed perfectly in any .30-06 length action without any modifications, while the 6.5/.284 often requires some gunsmithing to feed smoothly, especially in a controlled-feed action. Though the 6.5/.284 will normally feed easily in a push-feed action.
The 6.5-06 has just about the identical powder of the 6.5/.284, and is capable of the same velocities. It will also work smoothly, of course, in a .30-06 action, if going to the trouble and expense of a wildcat is worth that extra 125 fps.
