I wonder if what your trying to achieve or expect is worth the effort,
Your wanting better buffer, faster auto focus i assume from what i am reading, and to know if the D850 you have is defective or not.
The D850 like any other camera is only a tool that fits a purpose for what it was designed for.
If you have the slightest concern about the D850 you have on loan take it back and get another one.
If the same lack of expectations exist then there are several possibilities to consider
1) use the best possible CF EXPRESS card you can afford
2) make certain the camera is formatted and set up optimally, especially with focus and release settings that can adversely effect focus speed and frame rates.
3) question your technique and what objective you have in mind and how the D850 tool fits that purpose.
Or get a genuine grip and use the D5 D6 Z9 battery for that boosted responsiveness in focusing attack speed and increased frame rate.
4) ask your self about your set up , your technique, what your doing, and if its all correct, then ask if the the D850 is the right tool.
I find the D850 flawless used and set up properly, i use it with out the grip and with the Grip, with the grip i use a D5 D6 Z9 battery, a CF Express card, in my case the Cobalt only because i have them from the Z9, i do find with the grip on the D850 the focusing attack speed seems slightly more responsive, the frame rate is faster, the buffer exceeds all my expectations for that tool and i never clog up in the buffer even in raw Lossless compressed HC 9FPS because i don't with my technique spray and shoot endlessly, also that's not what the the D850 was designed to do, the D850 does respectfully meets all my expectations wildly, i have zero issues buffer speed or focusing.
With the D850 I do birds in flight, wild life, sports action like soccer, surfing, rodeo, basket ball etc.
The right glass can also effect performance as we know.
If the D850 doesn't do what you want it to do and you have accessed your self first as doing all things correctly and your using the optimal card then you have the wrong tool.
If i need super fast focus attack speed with a series of demanding repetitive HC burst after burst then I use a D4s D5 D6, if i need that demanding performance at 45mp raw Lossless then i use a Z9, as its the right tool.
I give all these tools the right cards, settings and GLASS to match the APPLICATION as i do equally the D850......Happy Days no issues what so ever.
Again I find the D850 is brilliantly capable as an all rounder do everything well tool, i have even replaced my D4s and D5 with 2 x D850 units, one recently sold as i now have the Z9.
If i find the D850 doesn't meet the needs in an application which is rare, its not the cameras fault, its mine because i am expecting something from a tool it wasn't meant to do.
We can analyze, search, question, compare and overthink or over complicate the situation till the sun sets, its all horses for courses.
Example: I mean i like the Z9 20 fps 45mp good tracking, but gee i still love the D6 as its a different tool that for me in low light super fast indoor sports action or in very challenging conditions the D6 focus attack, tracking and stickiness simply is better than mirror less in any brand, now its a tool that i find fits better for that purpose yet that doesn't take away from the Z9 or other mirror less brands, the Z9 it offers other benefits like 20 30 fps 45mp etc, excellent video, i respect a tool for what it was designed to do, and the D850 is the best DSLR Nikon has ever made, used properly.
Only an opinion
That doesn't answer my question in any way of course - like I said, I needed two numbers based on members' practical experiences of the D850 to determine whether what I was seeing was to be expected. I now have thanks to the more practically minded among us.
