Plus 1-2.5w per active cable. You need the heatsinks as the cx4 cards expect active airflow, and active transceivers as well.

I have a 10gbit dual port card in a Lenovo mini pc. There is no normal way to get any heat out of there so I put a 12v small radial fan in there as support. It works great at 5v: silent and cool. It is a fan though so might not suit your purpose.