I don't have the RF 1.4x and for now won't get it. Reasons are the very high price for what it is and the issue with the reduced range when used with the 100-500mm.
Also, I expect that cropping will give a similar result to using the 1.4x. It would be interesting to see actual tests, though, with the 800 and the 100-500mm.
Personally I would not get the 1.4. Not for BIF. With the 1.6 crop factor 700mm is plenty for that. The light loss alone. I'd use it for stills but you can crop that even more in post without it.






