I had been observing a lot of chatter about the performance of 3rd party lenses with the new RF mount. In particular the comments that the very popular Sigma 150-600c has focusing issues on the RF mount. A review HERE by popular Australian photographer Duade Paton in a video where he found focusing issues was amplified, despite his clear statements on post with interactions with me that this is only apparent at close to minimum focusing distance.
So, I decided to take that combination to a gannet colony close to where I live. Without doubt, the lens, used with the Canon basic EF-RF adapter, was not as accurate as the native RF 100-500 lens, which is an amazing optic. However it performed acceptably - according to my statistics I got about 73% with a lock on the Sigma, and 84% lock with the RF 100-500 lens - the keeper rates are as much a function of the speed of these birds rocketing along and photographer error, so it was pretty good at getting the shots.
So, here are a few of images of my experience.
The bottom line is that the lens is capable of generating acceptable results for those who have this lens and are not updating to the RF native tele lenses yet.
For some awesome photos with the Sigma 60-600 on Canon gear, see the following You Tube video showing images in the Brazilian Pantanal: Pantanal Pix
I did some major purging of equipment and have sold the 2.0x extender and have the 1.4x unit up for sale as well. At the moment I still have the 70-200 f/4 IS USM MkII (which I shall keep), the 100-400 IS USM MkII, the Sigma 150-600c and 60-600s EF lenses, but really only have retained the 5DsR body from my rather large menagerie of DSLRs.
I am moving fully into the MILC base, so 1x R5, 2xR6 and 1x R6MkII. Optics Rf 100-500, 24-105 f/4, 24-240STM and plan to get the RF 14-35 IS USM, which will give me all the range I need. I prefer to use the EF 70-200 f/4 IS USM MkII over the RF unit, as it is a great optic and does not extend with zooming, further protecting the sensor from dust.