I'd say you are one of the first people who actually figured this out. Boyd is the first. He just S---canned an RUM that couldn't hit the broad side of a barn in order to have a 30WSM that actually shoots. An efficent gun is a lot nicer on brass and expendables than a great big cannon that just eats components. Ask any of the RUM guys how many firings thay ever got out of thier brass... Most will say 4-8 and a select few may go as high as 10 but I'd have to see that to beleive it. Now, in a similar barrel length, you can shoot a WSM that will be within 10% in speed with 25% less powder and the brass will live forever. It'll almost never heat up because of the rate of burn. (Ask Boyd about that).
Go to the PA 1000yd club results and see how many matches were won last year with 6.5-284's. I was going to name names but I won't here. The point is, a majority of matches were won with cases that can be at least nearly filled. I have 1 paper on my desk now and no RUM's were shot in the heavy class that weekend. In the light class, 1 took a second in a relay, the others were none better than 9th out of 10 or 11 in a relay. Group of the day was shot with a 2.3" 30Cal case (I've made dies for it so I know the size) score of the day shot with a 6.5-284. Score shootoff won with a 6.5 and group won with a cub. Hmmmm. Where were the 3200fps 30's that day. This just happens to be one week that I have laying here for other info in the paper, but, you get the idea. Other weeks are similar. Personally, I'd shoot anything in north america with any of those chamberings and wouldn't think twice about them being enough gun.
I don't know if this would be your first gun project or not, but, my single advice would be, build a gun that's gonna be fun to shoot... That way, you'll shoot it a lot. A gun that doesn't get worn out was a bad investment.
Now, for Ackley Improved. Could someone tell me what got "improved"? Hmmm, a steep shoulder that inherantly makes donuts in the case neck and won't resize worth a damn. Added case capicity for a case you couldn't fill to begin with, all so you can pay more for class C or D dies and waste time fireforming cases so they'll shoot. Now add the turbulence generated at the case mouth because of the harsh angles at which gas has to leave and you get added throat erosion from the wash of the powder. I'll take the Unimproved version any day.
264 Win mag. Nothing more than a 7mmMag necked to 264. Entirely too much case for a 140 Grain bullet. The powders that will work in a 6.5 will be in the 45-55grain charge range and the case will hold 75 of about anything. You can use them for a deadblow hammer!
In most cases, you will cease to gain velocity no matter what amount of poweder you put in the case. This is especially true in large volume cases where you use a slow powder such as R25/22, H1K 870, ... My 416 gun would shoot H1K up to about 70 grains with 40.7fps/grain. After that, the speed increase/grain of propellant was about 18fps up to 75/76 where I could no longer fit any in. All I was doing was heating the barrel for no reason at that point. 4831 gives similar performance but has an overall higher top end velocity because of it's burn rate / efficiency in my gun. Sunday I shot 67Gr of 4831 with 210Berger's and got 2970fps avg in a 28.5" barrel. That's a Short fat case. No sign of pressure. I shot 70 gr in last years barrel but won't need as much now. My brothers 7mm WSM
gives better performance than that by quite a bit with a 180gr bullet. He gets 44.3fps/gr from 180's and 4831. Compare now the amount of heat being generated to a large magnum and immagine the difference in barrel and case life. In the hottest matches of last year I shot not less than 10 spotters and 10 record rounds. These were gone ALL in less than 9 minutes with the last 10 in 2 minutes and I grabbed the gun by the barrel and went to the weigh in every time. You could ask Boyd how he did for heat but his gun with 5 spotters and 10 record rounds was so hot he needed welding gloves for the weigh in.
Build a FUN gun. If it works, you're gonna throw all your other ones away. The cartridge doesn't have to be a ton of work to work well either.