R5+100-500+1.4x or R7+100-500?

And-roid wrote:

Honestly, you are fighting a losing battle with tc's already, these high f stops on the 100-500 already playing against it! Its alright saying its f10 equivalent on the r7, which ir is for dof, but physics are physics and the R7 gets a better f stop and can even be used at 6.3 and still out reach the the 100-500 with 1.4* on R5, it works out that you are shooting 1.5 stops slower, 1.75* slower stops equivalent!

Honestly, R7 is the way to go for 600+ and ef100-400 even more so!

There are real battle and there are imaginary battles.  Yes, a higher open f-number affects AF speed in very low light or contrast, and/or in large changes in focus distance, but the "lost light" argument is nonsense, regarding subject noise.  A TC always increases the area of the subject on the sensor as it decreases the light per square mm, and we all know that giving the same amount of light from a subject can give slightly less shadow noise at higher ISO settings, so, no, TCs do not increase subject noise, unless someone is so silly as to put a TC on that they don't need, and walk back farther from the subject, in which case the TC was a very dumb idea, for that subject.  If you're already as close as you can be and still want to make the subject fill the frame better, a TC does NOT increase subject noise.

There are other possible issues though, with TCs; slower AF with low light or contrast, and if your TC is not getting much more detail because of lens softness or technique, then a TC will exacerbate rolling e-shutter artifacts without getting finer details, because any sway of your lens or any subject motion will move faster through the frame.

So many things to consider!

I played around for the last couple of days with the RF800/11, RF2x, and the R7 just to test extremes, and I would say that the biggest problem with this combo (other than the great difficulty of getting and keeping subjects in the frame), was when I used e-shutter; it was often ghastly.  I have one frame that I saw in the review yesterday where a horizontal band about 1/8 the height of the frame was formed from what should have been about half that, making a bird's legs appear twice as long as they should have been, and the mudflat look dreamy in that band!  Something twitched, either me holding the lens or OIS or IBIS, and it completely ruined that shot.

Slow E-shutter + high magnification = trouble.  This is why I never got into using my Pentax Q with EOS telephotos for very small and distant subjects; with an adapter with a leaf shutter (most native Q lenses use leaf shutters), the rolling shutter of about 1/13s or 80ms was unbearable, handheld.