So, the final set of "in practical use photos"
My thoughts, after having done this rather unscientific experiment....and it involved an estimated total of fifty plus exposures, is, that for shooting at 400mm on FX, the lens is not going to be the primary factor in the final quality of the image. These are shot at 1/1000th at f/11, this to get some blur in the wheels, some degree of DOF, and to get the lens near the "sweet spot". This is not intended to be anything but a pragmatic example of the fact that is shown everyday on PAD. The photographer's technique is the important factor if at least minimal quality equipment is utilized for the job.
A little note as I looked at the Mustang...400mm f/2.8 vs, the 70-200mm f/2.8 +TC-20EIII...the latter is sharper in the 2000Px comparison IMO. However, as a car had gone on its roof, in another part of the track, a minute before hand, this exposure was taken with the car moving slower. Note the wheel blur difference.
Second note when shooting motorsports....it is helpful to have earplug speakers and to listen to race control. This lets one know what is happening when the track announcer may not tell all that is occurring at another part of the track.
Hope all enjoyed this.