Do you mind posting pics of the adapter you made? I'm usually afraid to make my own adapter for fear of sending the wrong amount of current.The cage has nothing to do with powering up the backplane. It's just a piece of steel to hold the drives in place.
The backplane needs 12V through a Molex MicroFitJr3.0 8-pin connector - I just made an adapter from the Dell original cable.
As for the speed I said it a few times already (and even posted a link to the PEX8632 spec) that it is PCIe 2.0. Expecting PCIe 3.0 support from a $25 card is a little bit too much.
The last post in this thread suggests it's a CPU limitation? Unless he's mistaken.