There is a very good article on the 6mmbr website that talks about a 284 win that a guy built on a short action. (gun of the week 95) His custom mag limited him to a 2.900" COAL and he was still able to get 2990fps with 162gr A maxs and RL-17. That was with a 25 inch barrel. With shorter 140's i would think you could still get over 3000fps depending on your magazines COAL limitations. I would think that you are correct about the 284 win having a little more puch, even when using a SA. The wider case will give you more powder capacity than the 7mm-08 with a similar COAL as a 7mm-08. Most data that I have seen shows a COAL of 2.800 for the 284 win so if you have room for a little longer COAL than that i would think you would be able to duplicate or exceed book velocities with the longer 160+ bullets.