I was cramming 73ELD's balls deep into .223 cases last night for a ladder test out of my AR. Then I read this thread and thought I would offer an unsolicited and inexperienced (yet completely free!) opinion.
When I did my .22BR, a .223 AI and a .223 Vanilla were also on the list, I had such a hard time making up my mind that I ordered both a .308 and .223 bolt heads for my Origin. I REALLY wanted 88's at 3,000fps out of a 24" barrel without leaning on it TOO hard. So, I went .22BR. If you run the numbers with that bullet at 2,800 (like, out of a .223AI with a 24" barrel), it will really piss you off. There's about another 1.1-1.2 mills of elevation at 1,000 yards, and a couple tenths of wind. Still supersonic to 1,200. That's it as far as differences that I can find (this was on Berger's calculator, at 1800' ASL). I'm still in love with my BR, but either iteration of the .223 will get remarkably close to it, real world.
Back to cramming bullets into cases to fit into AR mags (and these were ASC's!), the .223 case changes personalities when ran in a bolt gun with enough mag room and throat to seat the bullet out. The shoulder on a .223 case is only around .350", and when you cram a .224 bullet in the middle of it, deep, it REALLY eats powder room. Much more so that cramming a bullet deeply into a wider (.308) case. My opinion is that the .223 gains as much or more capacity from seating the bullet out than taking out what little taper the case has and changing the shoulder angle. Based solely on volume gained solely on improving the case, I had a hard time figuring out where the AI version grants more than another 50-60fps, all other factors being equal (and they rarely are).
The biggest game changer for the .223, AI, or other .22 centerfires in general has more to do with high BC bullets than capacity. I don't know that it would be worth fretting if we were talking about 55 grain Hornady blue noses (though it's a great bullet, for it's intended purposes). It's worth noting that stubby bullets don't eat up nearly as much case capacity to begin with.
I think a lot of the remarkable speeds that I've read regarding the AI version have more to do with leaning on the powder charge more that any large increase in capacity or magical case design/efficiency. Of course, your putting a little case into a great big bolt action, so load away. How soon the primers fall out and the headstamp disappears is strictly a matter of personal choice. I do think that the Ackley version does give a bit more ceiling on top end, what would be a very hot load in a .223 would be a BIT milder in the AI.
Bear in mind, a whole lot of what I just typed is based on what I've read and studied and not what I've done. My .223 loading and shooting has primarily been with my AR. A different animal entirely.
If I ever do spin a barrel up and use that .223 bolt head that's still sitting in the Bighorn box, I'll likely do an AI. Fireforming is, uh, just, shooting, so I'm not fretting that. It's not like using up barrel life is a big issue either way. One might want to work up a fireforming load that crams the bullet into the lands for a consistent form.
The AI is a dead sexy beast, and I could well use the extra .25". Well, my wife could. I'm getting to old to care one way or the other.......