I'm the one who asked before and I never did get any factual information regarding gas port diameters between the two barrel lengths. The gas system length is the same between the two, though. I've brought this topic up numerous times in different threads and I never get anywhere, but in order for the 14.5" upper to run as reliably as the 16" upper while using the same length gas system, it would have to come from KAC with a larger port. The backwards thing that I can't understand is why KAC includes an H2 buffer with the 14.5" guns and a standard carbine weight buffer with the 16" guns. As I mentioned, the 14.5" would already need an enlarged gas port over that of the 16" because of the reduced dwell time, so why they chose to go with the heavier H2 buffer on the 14.5" is beyond me, as this would require an even larger increase in port diameter versus just sticking with the carbine buffer from the 16".
In the end, I bought a 16" rifle and sold the factory upper off as new, then replaced it with a new 14.5" upper that I pinned/welded a MAMS to. Doing this saved me the hassle of buying a factory SBR and removing it from the registry, but now that I'm past that and have many thousands of rounds through the gun, I wish I would've went a different route, which I will explain below.
I do regret not shooting my 16" upper before selling it, just so I had some basis for comparison between it and the 14.5" upper that I replaced it with. I can say that the port on the 14.5" must be plenty big because I swapped my factory receiver extension out for an SR25 extension so I could run A5 components and not only will the 14.5" upper run the factory H2 buffer just fine, but with the A5 setup installed, it has no trouble running a Sprinco green spring and an A5H3 buffer. I've tested this at length, and the gun will run the weakest of ammo without issue with the A5 system, even in very cold weather while extremely dirty with over 4k rounds between cleanings.
All this testing tells me that the 14.5" uppers are over gassed from the factory. Don't get me wrong, they still feel nice and smooth, but not as smooth as they could be. If I had it to do over again, I would've cut the 16" barrel down to 14.5" and swapped the 14.5" URX4 out for a 13" instead of getting the factory 14.5" upper. I think this would've left plenty of room for a very reliable setup, but been even softer shooting.
I do have the necessary tools to disassemble the Mod 2 upper, so I will eventually get around to pulling mine down and measuring the gas port. I have been planning on doing this for months now, just haven't gotten the time to actually do it between work and then using the gun on the weekends. I will try to remember to report back with my findings for you and anyone else who is interested. It would be a great point of comparison if we could get someone with a 16" Mod 2 upper to do the same.