This message was posted on behalf of Intel Corporation
Thank you for posting in Wired Communities. The network adapter is backward compatible to the PCI-E 2.0 slot. As there are many factors affecting the network performance, it is recommended to use the network adapter to a slot that matches or exceeds the bus width of the adapter. Please refer to this website for reference:
It will work still if you are referring swapping the X550-T2 to the supported PCIe 3.0 slot in the future. Please contact your motherboard support to check if the X550-T2 is compatible to your board system. Hope the above information help.
Thanks for confirming that Sharon; I figured it was likely the case that a single 10G connector would work in a PCIe 2.0 x4 slot for now and then both could be used in a PCIe 3.0 x4 slot in the future as you've stated. My motherboard is older and I doubt that anything this new would be tested on it for compatibility.
Due to the PCIe 2.0's encoding scheme that is less efficient, each lane delivers 4Gbit/s each direction for a total of 16Gbit/s. As there is overhead involved, one 10G connector won't use all 16Gbit/s itself but both would easily exceed it and then some. I just wanted to be sure that the card would handle this scenario for now; with Intel's driver support and lifetime warranty it'll be good for many years to come when I make the upgrade.
Edit: 4x4=16; I used the original unencoded 4x5=20 (fixed it above).