Really where these Tinys are intended to live is under someone's desk, probably in or near the cable tray, covered by lots of thick electrical wires, turned on 24/7. So while I'd be (a bit?) alarmed if this was my personal desktop temperature I think you shouldn't be too concerned.
Another option, and I have not had any experience of this myself, is just to buy what Lenovo insists on calling the "base chassis" or "base assy" for the M920x or P330 but which is what you and I would call the top cover. You have an M720q, right? I think the FRU is either 02CW461 or 02CW663 and it has a grille over the CPU heatsink fan which should keep everything a bit cooler. For ~£28 delivered (in the UK) it's probably cheaper than you buying a nice fan and whatever other stuff you have to mess around with, your time, etc. This is mentioned in the original post (under "model upgrades"), and a bit lower down ("heatsinks") you could also think about the 65W heatsink assemblies which, even on a 35W TDP CPU, will most likely eject the CPU heat more efficiently and leave the overall chassis a bit cooler as well - particularly in combination with the increased vent-age? Just a thought, particularly as your problem is probably more due to the NIC in combination with the CPU's heat rather than the NIC by itself.
Here's a pic of 02CW663 - it "should" fit an M720q but <insert disclaimer here> :
Another option, and I have not had any experience of this myself, is just to buy what Lenovo insists on calling the "base chassis" or "base assy" for the M920x or P330 but which is what you and I would call the top cover. You have an M720q, right? I think the FRU is either 02CW461 or 02CW663 and it has a grille over the CPU heatsink fan which should keep everything a bit cooler. For ~£28 delivered (in the UK) it's probably cheaper than you buying a nice fan and whatever other stuff you have to mess around with, your time, etc. This is mentioned in the original post (under "model upgrades"), and a bit lower down ("heatsinks") you could also think about the 65W heatsink assemblies which, even on a 35W TDP CPU, will most likely eject the CPU heat more efficiently and leave the overall chassis a bit cooler as well - particularly in combination with the increased vent-age? Just a thought, particularly as your problem is probably more due to the NIC in combination with the CPU's heat rather than the NIC by itself.
Here's a pic of 02CW663 - it "should" fit an M720q but <insert disclaimer here> :