Before the rain could even stop this afternoon, I was out the door. My mission: to see if the RF 100-400mm + RF extender would exhibit any of the problems I saw in the bigger zoom yesterday. Working in intermittent drizzle and dim light, ISO soared much of the time, never dropping below 1250.
No issues were found, and on the way out with light improving, the combo captured just a bit more detail from a signboard that I also shot with the 100-500mm yesterday. Anecdotally I see no difference in ISO with the combo at f/11 and the big zoom at f/7.1. Autofocus only missed on a sunbird, with whom no setup can keep up all the time. Both shots below are 100% crops, first at ISO 6400, second at ISO 2000.
No need for a crystal ball right about now . . .