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.
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.