Hi Les,
Michael Reichmann made a similar comparison in mid 2015 for his Canon G3X review, which is available here on
Lula. His finding was that the smaller Canon at base ISO did hold its own against the bigger Tamron 150-600 he was using as a comparison for the test.
According to various reviews, the lens of the Sony RX10III performs even better at the long end, so chances are that you might be satisfied with the rendering, especially for hand-held shots of distant subjects. You might want to rent a Sony RX10III to verify by yourself.
Following Michael's review of the Canon G3X, I bought one and was very pleased with its image quality between 35mm and 450mm. Image stabilisation is outstanding, too. What annoys me, however, is that the AF could really be improved upon. The ability to lock on the subject decreases dramatically on the longer end, especially in suboptimal light. Manual focusing is a joke with only 4x magnification. Also, it cannot record at more than one FPS in RAW, which makes it a doubtful replacement for a DSLR & long lens for wildlife.
So you might be pleased by the rendering of the Sony RX10III but do also check if you can live with the limitations of the platform such as low FPS and poor to medium AF performance (contrast detection only).
Regarding AF, at some point in the future there will surely be a new model of the RX10 featuring the on-sensor PDAF present on the current Sony RX100-V. This could potentially make a good replacement for people who occasionally need long lenses for wildlife but want to avoid the huge investment of a 500mm prime and its associated bulk.
Cheers,
Fabien
PS: I can only praise Michael for his outstanding reviews. These lead me to make long-lasting investments which really brought new insights on what could be made with a camera (XPan, long FL with the G3X, etc.). The G3X is the ideal all-rounder camera when the photo backpack has to stay home.