I have found that my RPi machines on hard wired network can manage thruput at 98Mb/Sec. Connect two RPi machines on the same switch and run iperf. (client on one machine and server on the other). That test is only HDX but then so is streaming video.
You may have loss in your powerline connections. IF you are making errors the protocol will be making retrys and the total thruput suffers.
As some have pointed out buffering can be either in RAM or in SD or USB memory. Even though my network performs nicely I still allocate 20MB of RAM for buffer.
Tom S