Came across an interesting article on the effectiveness of reflectors on T5s. Includes a comparison of single reflectors (one big reflector over multiple tubes) vs individual M-shaped reflectors over each tube. Unfortunately, no study on individual round reflectors.

Also interesting study of the effect of painting or pasting backgrounds on the back glass wall. That will actually reduce the amount of light reflected back into the tank. So its better NOT to paste Oyama paper in you want maximum light reflection off the back glass. Instead, better to just put a sheet of paper/plastic and secure the edges.

http://www.advancedaquarist.com/2012/5/equipment