I have the rod mirror in the centre and it does restrict visibility quite a bit on tighter left hand bends unless you alter your head position.
I don't have strong views either way on having a central lookbackascope, but there's a very good reason to move your head when looking. If you just flick your eyes from position A to position B, your brain blanks out any image being processed while the eyes are moving. If on the other hand you move your head, the brain will provide you with images whilst you are moving.
You can try this yourself if you wish, but it's an important part of training pilots apparently. "Move your head or you're dead".
