What I've noticed more than anything else with this lens, it really prefers good lighting conditions.
Can't speak to a Canon tele lens, as I've never used one, but I guess that's where the price difference comes from.
When I'm in questionable lighting conditions, my keeper rate isn't that great.
On the other hand, when the lighting is good, I even open up the aperture a bit to give myself more DOF so in case the camera is not dead on, it's still getting enough of the bird in there to get a decent shot.
Here are a couple bird in flight examples using the 150-600 C:
View attachment 923066
View attachment 923067
Here the light isn't ideal. ISO is already at 2500 and the shadow on the face is not helping...
View attachment 923068
I would describe this one as a perfect opportunity missed. Lighting is perfect, but sharpness is still lacking a bit.
For one I was kneeling on the ground for a while, so I was tired, but I could have probably remedied it a bit if I would have stopped down to f/8 instead of wide open. ISO is 500 so lots of potential there...
View attachment 923069
Here is one that's not in flight, but on a tripod with remote.
Wide open at 1/800 and ISO 5000; with full frame camera.
Lighting again wasn't great, overcast and into the darker trees. f/8 would have probably helped sharpness due to DOF, but I prefocussed manually anyways, so not sure how much it would have helped. Would have negatively affected shutter speed or ISO regardless.
View attachment 923072
This one was a setup shot again. But I'm using f/8 maybe that's why the bird seems sharper.
1/800 and ISO 8000 as the evening light was pretty much gone. Again full frame and some crop.
View attachment 923074
EDIT: wanted to add that on the trip where I took the first four pictures, I attended a birds in flight workshop of one of the Sigma ambassadors and he said he "never gets out of bed below ISO 1600"
Thought that was pretty telling!