When I mounted my 300 f2.8 IS II/1.4xIII combo on the 80D, the results were not razor sharp. "Okay" sharp but not what I would expect from a very expensive setup.
I MFAed the combo and found I needed +10. Now the images are much better.
Why send your camera/lens to Canon when they will tell you how to perform MFA.
Download the pdf file.
MFA is explained step by step. Canon uses blades of grass to determine if your gear is front or back focusing. Easy-peasy!
One more thing to try...set camera on a tripod. Select a target that has contrast & detail. Focus on the target using Live View.
Then get out of Live View and focus again, keeping an eye on the lens focus scale. If the focus scale moves, even by a hair, then MFA