Yes manual focus, IS off, on my heavy duty Gitzo tripod and Wimberly head. Set the 10 second timer because you could see vibration from your hand when touching the camera.

You could test multiple ways, truth is this is probably pretty close to what I would be doing in the real world. For instance it is a rare day I would be able to have an ISO of 100 at the prime shooting time. It is common in midday when your doing something like Road Hunting in Yellowstone. The higher ISO would come closer to showing a failure than if you had great light and ISO 100.

I thought it was a fair comparison and here is why, a shot usually presents itself at a given location and distance at the exact same time. So a real life comparison would be the results you would get by cropping vs adding in the extender at the same distance in the exact same light. I could have decreased the ISO by going to a slower shutter speed, but in the real world I would seldom be under 1/500 and almost always shooting wide open.

I found it curios that the RF 2x handled the higher ISO so poorly. The EF 2x did much better. I have thought / or at least felt for years that putting the EF 2x on was not that much different the cropping. This goes back to my first big white a dozen years ago.

It might be overly critical to be shooting a target with a square that is .8 of an inch square and comparing it on a 32" monitor blown up to 8"x 8"