"naturally the 85Mbs rating of the powerline link is likely to be theoretcial only. I'm trying to figure out what a realistic bandwidth for this arrangement is because if it's only 15Mbs"
I'm using similar Power-Line technology too, my 85Mbps power-line plugs consistently give me around 65-75Mbps speed and they are sharing between 2 PCs. You may need to check that:
a) your Power-line plugs are plugged straight into your wall sockets or may through a more direct "extension adapter" and NEVER via a cabled extension pad (IF you have to extend your mains as well). Connecting your Power-line plugs directly on to your wall-sockets being used is the KEY to improve their bandwidths or speed as extended power-extensions would cause unmatched transmissions between power-line adapters, causing speed lost through errors during transmissions.
b) ensuring that ALL your Power-line Adapters are plugged into the SAME mains node, i.e. power-sockets in your house controlling by the same Circuit-Breaker or Fuse-Box. OR
b) alternatively, use 200Mpbs power-line adapters instead to improve further your speed.
However, improving options a) & b) are a must for either 85Mbps or 200Mbps adapters.
Intel E4300 OC with 2GB RAMs,Speedtouch 585 Modem/Router and Windows XP Pro
O2 Standard ADSL2+ Broadband!