According to one basic rule of internal ballistics, loading the standard .280 (SAAMI Maximum Average Pressure of 60,000 PSI) to 65,000 PSI (AAMI MAP for the Nosler .280 AI) will gain around 60-65 fps in velocity with loads in the 3000 fps range.
The .280's gain in powder capacity over the standard .280 isn't all that much, about 4 grains, which according to another rule, means a similar gain in velocity when loaded to the same pressure. So about half the .280 AI's gain comes from the higher pressure. Add the powder capacity to the pressure and the AI's gain is around 120-130 fps, in an equal-length barrel.
My first .280 was a custom rifle made by the late Dave Gentry, and I handloaded it warmer than the SAAMI specs, using both the old "pressure signs" familiar to most handloaders, and a computation of what should be possible when loaded to .270 Winchester pressures (also a 65,000 SAAMI MAP). It had a 23" barrel, and at first I used two different loads that shot to the same place, a "deer" load with the 139 Hornady Spire Point at around 3100 fps, and the 160 Nosler Partition at around 2900. The Hornady load shot very well, but the Partition load wasn't quite so accurate, though it killed everything I pointed it at, including one of my two biggest caribou bulls, shot at just under 400 yards. The first shot landed in the right place, but the bull (as caribou sometimes do) locked up and stood there, so I shot again. The second landed within two inches of the first, and the bull fell over. So it didn't really matter that the 160 didn't shoot quite as well as the 139.
I eventually was reminded by the late Chub Eastman, who worked as the writer-relations guy at Nosler for many years, that Nosler also made a 150-grain Partition, which in his experience tended to shoot better. Eventually I settled on it as an all-around load at just about 3000 fps, which shot and worked great. Of course, that was back before super-high BC bullets became absolutely necessary for killing big game.
Have owned two other .280s, and three .280 AIs, and never could tell any difference in killing power or accuracy between them, given the same bullets--these days often with much higher BCs. But then I always handloaded the standard .280 to what was probably around 65,000 PSI.
