The Powerline adaptors probably won't help much, wired ethernet straight into the router is best.
ThInk you need to experiment a bit. I normally just leave the jitter buffer on auto and stick to the preferred buffer delay. That typically gets me latency of around 25-30 ms for UK servers, 40ms for one in Frankfurt at an audio stream rate of 447 kbps. If anything, 2.67 buffer delay made it sound worse but did up the ASR to 760 kbps!