I only has one Mac to run compressor, so I am not 100% how compressor utilise the other network computer.
However, I doubt if Ethernet will hold it back.
1) the processing should be totally independent from the host once the required data loaded into the client.
2) it's designed for Mac, and Ethernet is already the best connection method natively.
3) I know some guys they buy a bunch of Mac Mini only use them as compressor clients. Never heard that the mini is held back.