So... I finally got my hands on a pair of what is currently top-of-the-line powerline ethernet adaptors.

https://www.tp-link.com/au/home-networking/powerline/tl-pa9020p-kit/

I also have access to a proper Ethernet test set, so I couldn't help myself. Some of you might find this interesting, so I'm sharing what I found.

With both adapters connected to the same quad wall power outlet (like, beside each other for the best-case scenario), its its app shows a 1501 Mbps link. Spirent TestCenter reported the following result:

Frame Size  Throughput                          Latency
bytes          fps                bps                 usec
64              180,431          92,380,672    2789.600
128            180,796        185,135,104    3229.697
256              90,579        185,505,792    2255.341
512              44,349        181,653,504    2339.398
1024            27,314        223,756,288    2639.932
1280           18,149         185,845,760    2547.818
1518           16,255         197,400,720    2658.047

Sent it to my FIL to replace his TL-PA4010, and who's now enjoying it with his 100/20 connection.