Good questions. I am not sure the answer really matters much.
The Colt 11.5" barrels I have seen have been very solid and stable. My opinion has been that the 11.5" is a perfect length for carbine gas system, especially with respect the dwell time. I have seen 11.5" barrels take any ammo, suppressed, unsuppressed and basically just work. The 10.3" barrels have been unstable since they were cut or factory made, and only with the right mix of ammo, gas ports, buffers and chambering has the 10.3" barrel settled in and working well. I just have not seen the 11.5" carbine barrel needing to have a special gas port size.
That said, I would expect that the best size would be something just smaller than the optimal size of the 10.3", maybe like 0.068", as the barrel has more time to emit gas before the muzzle end. Theory here. Perhaps Colt and others have found that a larger gas port somehow works better.
Way back when... the 10.3" had as large a gas port size as 0.080" and that was done to accept all sorts of ammo, especially the M855 ball ammo, but those two combinations did not work. If the 0.080" was the engineered standard, then I can see that 0.074" would be optimal for the 11.5" So, looking at it both ways: original engineering design, vereses changing the 11.5" based upon empiracal results from the 10.3"
Either way, unless you have a problem, who cares?