We have installed several d7o units, and they all work very well.
Access to the Manual control modes depends on yoru camera's ability to change flash exposure compensation. Most housings don't access this function, making it impossible to use that feature. You can, however, set the manual outputs on the flash.
The jury is still out on the d2x converter, I've only done one installation with Inon D-2000w strobes, and it requries a lot of exposure compensation.
I would enourage having someone who knows what they are doing install this for you, as soldering those teeny tiny wires isn't much fun, and replacing melted bits gets expensive. In the US, my company offers the retrofits, as does Underwater Photo Tech.