The Ackley rounds are a real chore.
You will learn to hate the fireforming process.
No. I don't see how loading the brass once and firing it in the chamber, also once, is a real chore. Last time I checked, most of us are here because we LIKE shooting guns. Besides, how often do you really need to make new brass? The improved cases last longer than the standard designs they are based on, due to improved shoulder angle and greater case capacity bringing lower pressures.
As the 25-06, 270, 280 and 30-06 all share the same parent case, there should be very little difference in the percentage of ballistic gain.
Again, no. Larger case capacities make the most sense and the most gains when used with heavier bullets, plus the larger the bore size, the more velocity you will get (within reason) for the same powder charge. Therefore, the .30-06 AI gains much more than the .25-06 AI. Plus, what is said is true, that with 165's and lower, there is really very little difference between the AI and its parent. With 180's and up, you will see a real difference. For instance, look at the specs on the .308 vs. .30-06 in the 150's and the 180's. Not much difference in the lighter bullets, very noticeable difference in the heavies.
BearHunter, the .30-06 AI is an excellent cartridge. I have taken deer very cleanly with it and really enjoy it. It certainly is a simple way to 'magnumize' a rifle that is already chambered in '06, though I wouldn't build one from scratch. I have also tried the 8mm-06 AI, and really liked that one as well. I have a friend who shoots and owns many different Ackley improved rifles, and the .30-06 AI is his favourite. 2900+ with the 180 is easily achievable, 3000 is sometimes reachable in some guns, though I would say it is rare. If you want to shoot lighter than 180's, don't waste your time or money.