Not approved doesn't mean it is harmful. It simply means it might not work! Some claim it is enough to tape just the first pin. Haven't tried it as I don't see that it would make any difference. Taping just "hides" the TC from the body.
Canon claims 50% reduction in focusing speed with a 1.4x III and 75% with a 2x III. In reasonable light I find the 1.4x to be just slightly slower, but nothing like 50%, more like 10-15%, and all 37 central focus points work just fine. And, there isn't a hint of front or back focusing, not a hint! In my book, a big advantage.
Super windy and over 20°C out. Very strange. Birds are all lying low. Still, I wanted to practice some selective focusing and this was the only cooperative model I could find. All single point, small, with Antishock=0. 1/125 at ISO 400, no TC.
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I had all of 6 seconds for these 5 shots. I have Rls Priority set to off, but it seems to be a bit buggy and sometimes works and sometimes doesn't. So I just wait till I see the green box and fire. I notice that with each shot it backs off just a tad and re-focuses.