Re: 6.5-06 Ackley or the 264 Win Mag
If you are going to go the 6.5-06AI route, I would recommend sending a pm to forum member elkaholic. He had a reamer made for a variant of the 6.5-06AI that he calls the 6.5 Sherman. He has had pretty spectacular results with that cartridge. If you are going to go to the trouble of doing the AI, you might as well go all the way.
While the 6.5-06 and its variants certainly have a significant efficiency advantage over the .264WM, I don't believe there are any other advantages. I have seen claims that the .264WM isn't as inherently accurate as some of the other 6.5's, but that has not been my experience so far.
From the same rifle, with the same barrel quality, and with the same care in handloading, I don't think there is a detectable difference. I have a 6.5x55 with a Shilen barrel, a 6.5-284 with a Krieger barrel, and a factory Winchester model 70 in .264WM.
The factory Winchester is shooting right with the other two in terms of accuracy (although I haven't shot VLD type bullets in it yet). The only difference from one to the other is speed and a little bit of recoil. All three rifles are shooting 140g bullets into under .5" three shot groups @ 100 yards.
One advantage of the .264WM over the 6.5-06AI route is simplicity. Buy a standard 3-die set and either Winchester or Nosler Custom brass and you are in business. No special dies and no fire forming needed.
As a 6.5 addict, I don't believe you can go wrong with any of the 6.5's. Finding the right one for you is simply a matter of matching your desired performance level and loading characteristics with a given cartridge.