I have a mid 2012 Apple Mac Pro. I also have an Apple 30″ Cinema HD Display.
To make this machine Metal compliant I replaced the original graphics card with a Radeon Pro WX 7100 graphics card. This card has 4 full display ports.
The cable that came with the Radeon Pro WX 7100 does connect to the Cinema Display and it does render an image, but it does not deliver the full resolution of 2560 by 1600, instead it is at 1200 by 800.
Your adapter was suggested to me on the Radeon forum. I ordered it from Amazon and it was just delivered.
Unfortunately, after hooking it up, I am getting nothing but a black screen. Before I return it, I thought I would see if there is anything I should try or know more about this situation.