Mac
First off I'm not a real fan of the 270, no one reason, but not one I'd like to have. When I compare trajectories of the 270 and 30-06, using heavy for caliber bullets, I see no gain over the 30-06. Second, the 280 (which I don't want either) is slower cause it was loaded to lower pressures from the get-go. I've read that from numerous sources. If the 280 were loaded to the same pressures, it would produce GREATER velocities.
To prove that, just compare your own readouts of the 338-06 vs. a 30-06. As the square of the bullet base goes up, given a specific pressure, so does the velocity. Think a 30-06 can push a 180gr bullet to 3000 plus? Yet the 338-06, with the same case can. The same logic would apply to the 270 vs 280 IF pressures were the same, and reloading manuals will not show them the same.
It's the same way they got '06 velocities from the 308, higher pressures.