This is ALL great advise East Wind for Flo. You covered it all and I bet there is a solution within your suggestions.
For those that are suggesting that Flo stop down his aperture that is certainly not warranted for this image if all equipment is operating correctly and the focus point is on the lion's head...
Having to stop down defeats the purpose of spending all of that money on a 2.8 lens in the first place?
This shot should be fine at 2.8 at this distance with a sharp subject an the rest of the image going soft, calling your attention to the king!
First thing you need to do is get your lens and camera up to the latest firmware. The lens has to be sent back to canon for the fw upgrade but I would say its mandatory. The image does look like the camera has front focused quite a bit. The front face of the rock seems pretty good. Overall I agree with others that a little post sharpening in DPP or photoshop would fix this image right up. At ISO 100 you can sharpen quite a bit and lot lose IQ.
Regarding DOF, that lens should be tack sharp wide open regardless. Stopping down some can help mask focus errors. Camera AF systems have improved but accuracy is not 100%. When you can stop down a little, do it to improve your keeper rate. I still have to shoot 6 shots or so and pick the sharpest one out of the batch.
Regardless of that the image here should be much more in focus. Send the camera and lens back to canon and let them iron it out. For those of us that are nit picky about focus there is a great tool called Reikan FoCal which will allow you to do your own focus calibration. I use it with all of my lenses and it works quite well. The pro version gives you nice charts and reports as well as the ability to run aperture sharpness tests so you will know at what aperture produces the highest resolution.
You have a great set up. I'm sure the fw fix will take care of the problem as it addresses some AF issues.
I have a 5d III and the 300mm 2.8 II.
According to test this combination should crate fantastic sharp photos, but I have really big problem that to get sharp pictures.
I normally use the center focus point as single focus point.
I realized that objets that are more than 10 - 15 meters away the hit rate is very low.
I did go through the AF menu from A-Z but I found no solution. AFMA made everything only worse.
I attached one picture of many, that are all no really sharp. ISO 100, 2.8 1/4000
Does anyone of you has similar problems. With my other lenses I never had such problems (70-200 2.8 II, 24-105 4, ...)