Last I'd seen these things researched in detail, most agreed that the ping lacked margin only in at most one direction, the client being able to queue skills (send its query) in advance and the server needing only send a confirmation that the skill could be used, thus costing the ping effectively only once per GCD. At that time, it did not appear that 0 and 83 ms (or, 33 and 116) ping, for instance, were .5 seconds of double-weaveable GCD apart, as they would be if the effect of ping was twice that ping value, without any margin, per animation.
I live very close to the new location of the XIV servers, which is about as far as possible from the former location while remaining within the continental US. Over that shift, I shed about 75 ms of ping. The difference to my double-weaving, however, was less than .1 seconds, on frame testing. Between other factors in typical instanced gameplay, the server move had virtually no effect on what GCD I could double-weave at.
If there are more recent studies, I'm glad to hear them, but I never said anything about... anything taking 3 seconds. Neither 1899 ms nor 1999 ms are... 3 seconds. Am I missing something else here?
I'm assuming I did and am reading through your link now, which I'd somehow missed before.
Oh, and to answer the first question: The tests were from Bluegarter and Dervy, with process documents only from the latter. I suspect I slightly misremembered the era-context of the tests, however, as there were apparently changes reported near the end of Heavensward due to alleged speedhacking on skills exceeding the GCD down to a GCD's time (most noticeably on BLM, though I suppose that may have also affected double-weaving in some way(?)). Either Dervy and BG got it wrong or things really did change at that point and their and my tests were simply done prior and are therefore now obsolete. In either case, though... damn, double the effect of ping. That's... pretty harsh on the vast majority of players. I have to wonder if there's any way they can preemptively query the server to give some safety margin when queuing skills over which ping wouldn't affect us.