I am a fan of actual real-life testing vs. charts etc. This is especially true if you have the gear in hand to test. As an example, if a camera gives me quality decent enough for my uses (whatever those uses are) then that camera is decent enough for me to continue to shoot with.
The decision whether the camera is good enough for my use is predicated on the final product which I can hold in my hands and view or view on a monitor at the size that I would normally need it.
If that quality is sufficient, then I have no problems with the camera, despite whatever charts or extreme enlargements show me.
OTOH... If I don't have the camera in hand, then various tests that are published "just might" give me a basis for choosing one camera over another.
Here is a shot that I did in Hong Kong using a Canon 30D. At that time, I was shooting with a 30D and a 40D. I used the 30D at ISO 1600 with some noise reduction (this was years ago and I am not sure if I was working with NIK at that time.) Provided enough quality for my needs. I could not use a tripod because I was shooting over the heads of a crowd of spectators. The shot was hand held at f/2.8 using 1/45 second at a focal length of 44mm.
I am now shooting primarily with a 7D and a 7D Mark-2. This was an experiment using auto ISO at ISO 1250. The quality after some global Dfine noise reduction is plenty decent enough for my use..
I didn't need to go that high in ISO. I shot with a 1/1250 second shutter speed at f/8. The IS capability on this lens could easily have allowed me to shoot at a much slower shutter speed and the lens performance wide open is quite good. My dog was stationary so shutter speed did not impact her sharpness. I could have easily captured this scene using ISO 400 and very likely could have done O.K. at ISO 200 of even slower... ISO of 150 or so could have given me an exposure of around 1/300 second or so at f/5.6... I know that I can easily hand hold the camera at 1/300 of a second using 400mm. And I know that image quality of the lens is still very decent side open.