For wildlife and sport I've swapped from the D850 with 150-600mm and 70-200mm f2.8 to the Olympus EM1 Mark II with 40-150mm (80-300mm eq) f2.8 and Panny 100-400mm (200-800mm eq), and for this past weekend at the Moto GP borrowed the Olympus 300mm f4 (600mm eq).
In terms of autofocus the EM1-II is more than capable and right up there with some of the best DSLRs, only when the light is really bad does it start to struggle a touch. In terms of sharpness I can't tell any discernible difference at 'normal' viewing sizes. The area where you do see a difference is in 'pop' due to the lack of shallow DOF with m4/3 (relatively speaking). This isn't an issue for panning sports shots obviously, but if you want that pop that you get from the 70-200mm f2.8 or 600mm f4 on FF you're not going to get it. Likewise for the 'portrait' type shots you're not going to replicate that 'look' you get from an 85mm f1.8 or f1.4 on FF.
I'm really happy with the m4/3 though, having a 500g body and 900g 200-800mm eq FOV lens rather than a 1kg body and 2kg 150-600mm is a godsend. For me the weight saving more than outweighs the lack of shallow DOF. I could never afford, nor would I be willing carry a 600mm f4 lens anyway. The Olympus 300mm f4 (which gives you the look of a 600mm f8 lens on FF) is real quality and 'only' weighs 1.2kg.
Here's some of my recent efforts with the EM1-II and various tele lenses. Not the best images you'll see, but I'm no expert
Generally they look better viewed on Flickr.
1. 40-150mm f2.8
P6302247-Edit-Edit by
TDG-77, on Flickr
2. 100-400mm shot through fencing
P7073523-Edit-2 by
TDG-77, on Flickr
3.100-400mm shot through glass
P7145007-Edit by
TDG-77, on Flickr
4. 100-400mm shot through fencing
P7145550-Edit by
TDG-77, on Flickr
5. 40-1500mm f2.8 shot through fencing
Marc Marquez by
TDG-77, on Flickr
6. 100-400mm
P8248445-Edit by
TDG-77, on Flickr
7. 40-150mm f2.8 shot through fencing
P8246817-Edit by
TDG-77, on Flickr
8. 300mm f4
P8248996-Edit by
TDG-77, on Flickr