That's a tough question to answer with any genuine scientific basis. I can only address this from three decades of empirical observation. This ability to observe accurately is heavily clouded by what I like or have had extraordinary good luck with in past practical application.
Example:
I know I shoot better suppressed. Do my rifles shoot better suppressed, or do I shoot better suppressed? My guess is the likely answer is the latter.
There are tons more, some that may border on superstition. There's an ejection pattern I like, a recoil pulse that I like, a balance that I like, products I like, manufacturers I like, etc. So, you probably should take whatever I say with a grain of salt.
To address your question: I like to use the longest gas system that a barrel length seems to like - i.e., function reliably and shoot accurately.
In .223/5.56 NATO, to me that means:
10.5" - carbine length
11.5" - carbine+1"
12.5" to 14.5" - mid length
16" - intermediate length (either KAC or Noveske/Kinsel)
18" - rifle length (as long as I go)
You can't talk about gas systems without also considering recoil systems. The older Vltor A5 and now the newer BCM MK II recoil systems make a world of difference and are all I use unless the customer absolutely insists on something else. Why? See the top paragraph, above.
Throw in an adjustable gas block, maybe an ideal low back pressure suppressor, and you can really get things just right. Ok ... what you THINK is just right.