I do have two lenovo machines (an m720q and an m920q), both running connect-x 3 single sfp cards. These aren't insanely hot, drawing about 5 watts more than the mini PC without any card installed. Only downside for the cx3 is that it doesn't support lower ASPM states. You would need to pick cards like the connectx-4 lx 10g cards for that.
This is probably the mostly known fan shroud mod that
@Parallax mentioned, it's quite easy to print and does help a lot even with a 4010 blower:
A fan shroud to mount a cheap 4010 blower fan into the Lenovo tiny series, in order to cool off high perf network cards. | Download free 3D printable STL models
www.printables.com
Before printing it, what I did with moderate success is cutting open the side of the cpu's fan shroud, allowing some air to exit to the side to cool the card.
It's not pretty, but it did work quite well. The air is channeled just with some well placed pieces of masking tape, with a bit of airflow from the outside the card stays below 70°C, usually in the 65-68°C range. With the baffle and fan it's a whole different situation, the other machine I'm running with it is at 56°C. This at fall/winter room temps (20-22°C).
I would like to upgrade my network interface (i350-T4) to an interface with SFP so I can connect it directly to my ISP AON point.
Do you have an SFP ONT? make sure it does include the ethernet interface, some ISP do use SFPs that won't work outside their routers, as these have some non standard stuff inbuilt needed to interface the SFP mini ont.