I don't think any animal you hunt will be able to tell the difference. Nosler makes some 150gr LRAB's for both calibers which carry high BC's and perform excellent. I got a pass through on a moose at +/-250yds last year with my 280ai.
I am a 280 fan, it started out with a 280rem in a win model 70 featherweight. For some reason the gun was just a flat out killer, deer, elk, moose, it didn't matter they all dropped dead from a single shot at ranges up to 600yds. Then, like you I found a Kimber in 280ai and bought it, haven't looked back since. I'm on my 4th 280ai now and don't see myself moving to another big game cartridge any time soon. With the advancements in bullet construction over the past few years with the likes of the LRAB's and the ELD-X bullets, I'm good to shoot out as far as I'm comfortable with my 280 and I'm confident my bullet will perform.
I say if you want a 280ai, go for it. It's my personal favorite cartridge, but as much as I love it, when it comes to real world performance I don't think the extra 0.007 of an inch is going to make much difference on the animal. In my head is a different story though....
Disclaimer: I am not a "Best of the West" hunter and I have no intentions of shooting anything past 600yds. Besides, I don't have the optics to be able to judge an animal past 500yds anyway.
