Great news you got it working. But I would chase up ASRock, they should be helpful if they do say ESXi is supported.
I do think your days are numbered with old MBR layout. I'm surprised ASRock never tested this, insert a DVD, install, boot....simple...how did they miss this?
I'm not sure VT-d would be interfering with your PERC controller. Try changing PCIe slots as each CPU controls different PCIe slots.
You mentioned Direct I/O was working, what did you pass through?
Do you know the stepping of your E5 2620, C2 is the one you want, fixes many VT-d issues. If you know the, sSPEC. If you see SR0H7 printed on the CPU chip, C1, SR0KW, C2.....but if you have something different, do let us know ;-)
I think you have a really good rig!