I fly with my Kifaru 14’r too. Loaded to the max. It fits in the measuring box at Gate check. Best pack for travel/ hunt/ vacation I have found yet that fits in overhead carry on. I have used it on Delta, United, Jet Blue, Alaskan, and American.I believe every airline actually publishes the dimensions of the maximum size carry-on they will allow. Like you, I've wondered what I could get away with and not the published numbers. That being the case, I had a Kifaru 14'r on a flight a while ago with no issues. The pack has 23-inch stays and is quite a bit larger than published dimensions for the airline I was flying with. Maybe a case-by-case situation...
People WAY overthink this. IMO unless you are on a full flight/board late, really push the luggage size limits, or are unpleasant to the airline staff, you should not have an issue.