Hi Anil ji,

I'm not familiar with the lens, but f8 and 1/800 would usually be enough for a sharp shot as far as hand-holding goes. The berries look perfectly sharp to me, and the eye looks reasonably sharp.
I am wondering if it is the +2 EC that has caused the issue here. I have a 760D, and I do not expect great details when shooting at high EC values - I use it only for record images.
Also, you have shot at ISO 800. In case your camera has any level of high-ISO noise reduction turned on, it might wash out some of the shadowed details (such as the bird's underparts) and appear to reduce sharpness.

For your other points -
1. Even I use the central point + recompose technique, I find it to work well (especially for the entry-level bodies).
2. Back button focus - I avoid that as it causes me to miss way too many shots. I am not aware of it reducing focus-hunting(?)
3. & 4. I doubt these are causing the issue.