Visible crew/equipment: In the beginning where Amanda Waller is talking in a restaurant. While the camera focuses on her towards the end of the scene, they move the camera right, facing towards her left side of her face. Watch the wine glass, you'll see the crew moving along with the camera.
Visible crew/equipment: When we see Floyd's past, he is on top of a building, lining up to assassinate someone and negotiating his fee on the phone. On the left middle, under a blanket, or something, there is another person on that roof when there shouldn't be.
Answer: Exactly as you said, to show off. He wants them to see he can fire just as accurately at an angle.
lionhead
I am not a gun expert and attempts to search this to verify my point have failed. However, it seems to me that he rotates the rifle to use the secondary sight. While this may be showing off, I assumed this secondary sight was intended to be used for a different range of target, or made a specific type of target easier. When he rotates the gun, it seems he is switching targets. Research has shown that the gun does have 2 sights (DDM4 MK18).
The DDM4 MK18 Deadshot is using has a single red dot sight, nothing "secondary." A gun has only 1 sight, perhaps also a scope, but nothing on the side that requires the gun to be used at an angle. Firing a gun at an angle seriously reduces the accuracy. If there is something attached on the side it's either a flashlight or a laser.
lionhead
If you re-watch the scene, there are very obvious iron sights that he is using that are accessible by tilting the gun sideways. Holding it normal gets you the scope, sideways gets you the iron sights. Not sure what the benefits would be but there are absolutely 2 types of sights on the gun he is using.
oldbaldyone