JFYI, for measuring power draw, you might be able to use `macmon`[0] to see the total system power consumption. The values reported by the internal current sensor seem to be quite accurate.
Speaking of hardware, the RTL8159 (10Gbps) hit the market late last year and is said to consume only about 2–3W. It apparently runs very cool compared to older chips. (Though it would need to be bonded to reach 25Gbps ;-)
I got me one of these adapters (RTL8127AF TXA403, with SFP+ cage); I haven't properly benchmarked it yet.
There's no driver support on macOS, and for Linux you'd need a bleeding edge kernel. Just trying to physically connect it (along with a connected SFP28 transceiver) to my Mac's Thunderbolt port using an external PCIe-to-TB adapter, macmon tells me a power draw of around 4.3 W, so it's not significantly less for half the bandwidth, but the card doesn't get hot at all.
[0] https://github.com/vladkens/macmon