@WANg yeup, I know you had a problem with VFs with the Twinville card (X520), which I also agree might have something to do with how the BIOS is playing with the OS. It is also the same problem I had with the M720q, albeit you got alot further that I did (CentOS just refused to allocate the VFs, but had no problem doing a pass through of one of the of the NIC ports directly to a VM). The Fortville cards (X710) are supposed to have much better support for VF NICs via SR-IOV, as well as better guest VM support as well (i.e. there's a dedicated linux driver for the VF NICs that looks like it's updated almost every two months.).
I think csp-guy had tried the X710-T4L which is an inferno so never got far enough, but the SFP+ cards like the X710-DA2 have a much smaller thermal envelope, especially when paired with DAC.
The power of a Fortville, and Columbiaville, or at least why I'm a fan of them, is their ability to use the card like a virtual switch via SR-IOV and Flow Director. For example I can get almost 50Gbps-80Gbps between two guest VMs with SR-IOV NIC ports from the same X710-DA2 card, with no extra software or configuration, and essentially zero CPU overhead. Using OVS, which is pretty much standard across the board, will max out at 25Gbps while chewing through CPU cycles. This means even though the hardware on the ports is 10G or 40G, it's possible to max out the PCIe bandwidth as long as you're routing between VMs on the same box.
Right now at work, the devs are using this feature (albeit on beefier Tyan boxes) to automate load testing of apps without being network bound. At home though, you could theoretically simulate a SAN or a cluster environment, without any CPU overhead, and everything contained on one box. I personally want to get DPDK working so I can route >5Gbps, without using TNSR (since PFsense/BSD maxes out any CPU at ~5Gbps).
The one option I hadn't tried, and not super keen to try either, is to recompile Debian with additional BAR space, bypassing the BIOS limitation, to see if it fixes the problem in M720q (Jeff Geirling actually gave me the idea based on his RasPi issues). If the T740s don't have this issue, I'd totally switch (also because I've been a fan of AMD embedded APUs ever since I got my first ALIX box from PC Engines and loaded PFSense on it)