My experience with the 58/1.4G is that it is quite sharp at mid to long distances even wide open, but in the near distances (say less than 2-3m) it does get softer at f/1.4, and some stopping down in the near range may be used to alleviate the situation. This is what I would recommend.
Although the D5 autofocuses the f/1.4 AF-S lenses generally excellently I do find that with the 58mm in the near distance range (close to minimum focus distance) at f/1.4 the focus isn't very consistent from shot to shot. At a distance of a few meters, no problems.
I think Nikon designed this lens to have some classical portrait lens characteristics especially since some users used the Noct 58/1.2 on DX as a portrait lens, so out of focus rendering is emphasized and sharpness not as much, when using it in the near distance range. And then it turns into a night light lens towards longer distances.
What I love about the 58/1.4 is the beautiful colour rendering and behavior in backlight which is consistent with the others in the nano coated f/1.4 series. It does have bit of tendency for cat's eyes as does the 105/1.4.
If you want to shoot sports (e.g. vaulting) with the 58/1.4 then I think it would work ok as the horse is quite big and we're clearly outside of the near distance zone of this lens. However, with these lenses you aren't going to get quite as fast focusing as you would (say) with a 24-70/2.8E. If the vaulting is indoors then the low light can also contribute to some focusing errors but the D850 should be as good as any in that respect. If it looks like it's needed, don't be afraid to stop down to f/2 or f/2.8; it's not like the "look" completely disappears. I quite like the 58/1.4G at f/2.8 by the way. You could also compare to 60 AF-S Micro to see how they compare at f/2.8; the 58 has less vignetting at f/2.8 but both have nice looks to the image.
Focus technique is an important topic for any sports photography situation, but especially if you're shooting at f/1.4. My favorite AF area mode is the 9-point dynamic in the D5; on the other hand if you have low light, and don't mind the larger area, group area may perform better but if you are shooting a horizontal image of the horse, then you may not be able to move the group high enough for the rider's head. In the vertical orientation this is easily done with group area. With dynamic area one can move the primary point to the edge or even corner of the focus sensor. However, dynamic area requires a certain amount of precision in using it as the camera basically assumes that you're pointing the primary point always on the subject and only assists briefly with the other points if there is nothing to focus on under the primary point. But if you keep this in mind dynamic area can work really well, especially the smallest one (9-point). Perhaps I would try 9- or 25-point for horizontal shots with a rider and group area for vertical shots.
Do remember that you don't
have to shoot at f/1.4 even if you
can.
You might find that there is an intermediate setting which gives the best look to an image. (I tend to shoot way too much myself with 1.4 lenses wide open and seem to have forgotten that excellent results can be obtained also at f/2.8.)
