UPDATE: I tried one more time this time on bare metal - the PCIE 4X connector indeed works with the QNAP 4 port card.
It seems ESX has trouble with the PCIE slot, but native is OK.
Now I have a 12 port router w/ 8 x 1Gbe and 4 x 2.5Gbe.
I'm going to test stability over the next few days.