One thing that I can't understand is why drive mode can't be assigned to any custom function; it is on the dial by default, and there is a setting on the dial that allows assigning it to another button in the lower back under the back LCD. But it can't seemingly be assigned under a custom function button or under the recall shooting functions feature. Almost everything else imaginable can be assigned.
I would like to have the following configuration:
A) Normal operation for moving subjects: AF-ON activates autofocus (AF-C), shutter button takes pictures. Exposure set so that I have aperture prioirty and auto ISO with minimum shutter speed of 1/1000s. Exposure compensation set using main command dial (easy exposure compensation). Drive mode set to CH or CL.
B) Recall shooting functions assigned to Fn1 (middle front button so it is easy to reach either in vertical or horizontal orientation). The functions set are exposure mode manual, shutter speed 1/200s, and auto ISO. I would then prefer to have the drive mode temporarily switch to Qc mode so that I can access a low frame rate with EFCS. This would be used when the subject is still.
If the animal is standing, I can choose between medium ISO with slow shutter speed and high ISO with fast shutter speed basically by selecting A or B. I will usually choose B if I have time to think about it. When the animal starts moving quickly, I let go of Fn1 and turn the mode dial to CH but often the situation is progressing so quickly that I miss the beginnings of the fast movement. I don't want to fire at 14 fps when the subject is still as it would be wasteful.
Anyway this is a minor complaint. But it is major in that it affects how many shots I end up with from an encounter with a subject and especially how many repetitive shots I get, which just take hard drive space and time selecting the one shot, when there are often multiple identical frames. I guess a reasonable compromise is to use an intermediate drive speed that would allow me to minimize the extra frames but still capture key moments of the action.
I've now concluded that Qc with EFCS does give higher sharpness with the 500 PF used at medium shutter speeds, and that it's worthwhile to use when the subject is not moving or is not moving quickly. With the D850 the difference due to EFCS is perhaps more obvious but it's still clear with the 20 MP of the D6, after gathering enough material. Qc in the D6 can be adjusted to 1,2,3,4, or 5 fps so it can be customized to the user's requirements. I'm very happy about this feature. As with the maximum aperture of the lens being f/5.6, I'm not easily able to reach 1/2000s without high ISO degrading image quality, so I'm glad that I'm able to optimize image quality at slower shutter speeds. With shorter focal lengths (e.g. 70-200mm) I can more easily reach fast shutter speeds and the camera shake from mechanical shutter use is less of an issue.
I think I will submit a request for Nikon to allow assigning drive mode settings to Fn buttons via recall shooting functions. It shouldn't be outside of the reach of a firmware update.