2- Why the buffer should be of any concern for the D850? If the buffer is a factor in the shoot then use a D5 or similar.
A high resolution camera with high fps can be useful for sports photography as well as wildlife. If the buffer is small, this dampens the practical value of the concept of having high resolution and high speed in one camera. Thankfully it seems with 12-bit NEFs it is sufficient for most purposes.
I give an example. I often shoot figure skating. To keep ISO moderate, shutter speed high (the skaters can spin very fast), my first choice is the 200/2 provided that I'm shooting from a position close to the ice. At f/2, especially if the skater is close to filling the frame, I get a nice ethereal blur of the background and avoid excessive disturbance from backgrounds but still it is recognizable that there are spectators, judges etc. in the background. Sometimes the skater's arms and legs are extended like this
https://www.flickr.com/photos/ilkka_nissila/36953629383/in/dateposted-public/and sometimes the skater is small
https://www.flickr.com/photos/ilkka_nissila/37575916146/in/dateposted-public/I need to crop the image to accommodate for these variations (and different distances). I don't have enough space to use a supertelephoto zoom like the 200-400 (nor do I own one) and that would push me from ISO 1600 to 6400. So the option offered by a high resolution camera such as the D850 is attractive. It would allow me to crop from the images from the prime and have a bit more detailed images than if cropping from D5 images.
Is high fps for extended times necessary? No, it is not, I mostly shoot single shots but if I want to get jump sequences of a particular skater
https://www.flickr.com/photos/ilkka_nissila/30275677831/in/dateposted-public/then yes, high fps can be very useful and the sequence can last several seconds. In fact there are quite many occasions where one might use high fps, though I'm not a big fan of such an approach I recognize that sometimes it's necessary.
As for how much gain there is from shooting with a high resolution camera vs. intermediate resolution in such applications, well, it's to be determined, I feel the details in the dress seem more crisply defined with smoother transitions when using the D810 than when using the D5. However, on the other hand the D5 images are less noisy at high ISO.
In many action situations when working with a prime lens you may find the subject changing distance and the photographer is unable to compensate, so there is cropping in post-processing to get the best final compositions. And even with a zoom lens you still want some space around the subject to give some margin of error. The fastest focusing and highest quality supertelephoto lenses are still primes.
Of course there is the disadvantage of large quantity of data that results from high fps capture of high resolution images. However, one might use such technique only for a particular athlete of special interest and then the quantity of data would probably not be an issue. The other issue is balancing between SNR and resolution; at low to medium ISO there is no question high resolution cameras can give more detailed images. At high ISO the extra details may or may not show from the noise, and this is something interesting to see from D850 images. Focus accuracy can also play an important role in the outcome.
Generally I find 20-24MP is well suited to my photography, even though I recognize more detailed images can be obtained using higher resolution sensors; I spend too much time editing and want to reduce that time. However, if the goal is the best possible quality in a specific task then I wouldn't be surprised if the D850 came out ahead in many cases.
3- I received my MB-D18 plus battery at the beg of the month and only tried it on to check the balance v the D5. I still prefer the D5 for balance.
Without doubt. However, I believe the majority of photographers prefer to have the option of a smaller camerawhen the grip is not needed. I personally use the D810 without grip when I'm shooting landscape etc. but mount the grip usually when photographing people as I typically shoot a lot of verticals with telephoto lenses. However, if I am tight for bag space and especially if it is the second camera I might leave it without the grip.
