I made the long trip to visit Kirby Allen last month and shot a few of his rifles. While I am a newbie at long range shooting before visiting him, (the longest that I shot before was 350 Yds) I was able to hit the steel target that was set at 685 and 868. See this thread: Thank You Kirby!
All that I did was dial up or hold over (mil-dot) the dope that Kirby fed me and the guns did the rest.
For firing these guns for the first time, and only running a few bullets thru each gun, they performed beyond my expectations.
While I didn't shoot the .338 AM, from what I've read it makes a great 1000 yd. gun. I believe Kirby put a bullet thru an elk at 900+ Yds with it?
I settled on the 7mm AM, but I expect to do more deer/antelope hunting with it than larger critters. If I were to be concentrating on larger animals, especially ones that might bite back, I would want something that would poke a bigger hole in them. I don't know how the .375 would fare at 1,000 Yds, but it was pretty impressive at almost a half a mile! The only downside (for me) is that it was too heavy to consider it for a walking around rifle.
The only reason that I'm not mentioning any of the other 'smiths that you referenced is that I have no first hand experience with them.