So, we are talking four different cartridges here. For all intents and purposes,with the 40gr bullet, I would keep the .204 and 223AI as a comparison and the .223 and 20P as a comparison. The .223 and 20P have the same case capacity. The .204 and .223AI have an increased case capacity compared to the othe other two. So, when talking the 40gr bullet, I would agree, that in the .223 or 20P, you see little or no difference in performance. But, when you go to the .204 and .223AI, yes, I would say you are now in a different game with the 40gr bullet. I run a 40 vmax now at 3600fps out of a .223. I would expect to push that a little better/more with the .204 and .223AI, because of case capacity. Hopefully this makes sense. It does in my head anyway
. Now, with respect to the B.C. between the two 40gr bullets, you will see a bit of difference I would suspect, but I didn't verify the BC's.
Having said all this, I am currently working on a .204 build. I like the 20P concept all around, but for no other reason, I'm going .204 just for the security that it is a factory offering. Meaning that, I do reload, but if I ever get caught short, out in the middle of nowhere, forgot my box of reloads, ran out, lost all of my bullets in a river, ect... I could possibly find some locally to not completely lose a hunt or a trip.