One of the upsides to the Improved chambers by Ackley, and then others, was to be able to get velocity gains on by "simply" fire forming in a improved chamber, and, being able to still utilize factory ammo in a pinch. It is interesting however, that as an shooter of Improved cartridges and factory for that matter, I have never run out of ammo, while hunting or otherwise. But it does make it easier to obtain your brass.
Your wise and miserly to save your bullet money however utilising your breakfast of Cream of Wheat. My fireform load consists of 10 gr. of Bullseye, and enough COW to go 1/2 into the neck, then instead of wax I use a pc of tissue paper pressed down firmly. After firing it produces a perfectly formed case.
As far as increase in performance, an outstanding, yes it does! The difference in "water capacity" is about 2.2 grains, which equates to about 7%. You should be able to increase your charge weights from published maximums for the standard 223 to about 1/2 to 1.5 grains, of course depending on powder used. So with that said;
Yes I have a Rifle chambered in the 223 AI. (also a 257 roberts and a 338/06) Remington 700 VTR, it has a 22" barrel with the 1:9 twist triangle barrel and goofy muzzle break, to me all muzzle breaks are goofy. My rifle was rechambered by Wayne York of Oregunmsmithing. Wayne knows what he is doing, as it was done properly and headspaces so that I can fireform factory brass to AI with out excessive headspace prematurely stretching my cases.
Apples to Apples comparison utilizing the Sierra 65 gr SBT gameking bullet, Ramshot TAC powder, and Remington 7 1/2 primers, using the above parameters for a powder charge yielded an increase in velocity of 130 fps. I gained a bit more with the Hornady 55gr V-max but it was using 2 different powders, 1ea for the standard and AI, so more like apples to oranges.
In my opinion you cant go wrong with any AI chambering and yes, each cartridge is a test on its own as to what you may gain, and you will gain, something. Also by having that 40 degree shoulder your case trimmer, well, just sits there. Also, while any factory cartridge is good enough to take care of the business intended, one could argue that you really don't "need" an improved or wildcat cartridge. Then again alot of us really don't "need" anything, But it sure is nice to have the options!
Crash87