It depends on what adapters you are using. There is normally a tool available that will query the adapters to find out what line speed they are getting.

In theory if your adapters are HomePlug compatible (the Netgear 200mbps ones are not) then many of the tools that ship with various adapters should work. The HomePlug AV Utility that I am using reports raw line speed and also "coded" speed, the second of which I believe is equivalent to quote Ethernet line speeds. There is around a 25% difference between the two.

This means that even at 85mbps the maximum raw data speed you can expect is around 65mbps, which is around 8MBps. If you were using ftp you might expect to see close to 80% of the raw data speed, so with an optimally working 85mbps you might see 6.5MBps ftp speed.

Windows network shares seem to run at about half the speed of ftp, so you might expect 3.5MBps. So assuming you were talking about Windows networking you are probably achieving about a third of the optimal maximium speed.

Some of my estimates above are probably a little rough, but hopefully they are roughly in the right area.

The only advice I can offer if you don't have any choice on sockets is to unplug everything else in the house and turn one thing at a time back on until you find any noisy things. When you find the source of the noise remove it or try plugging it in elsewhere.

In theory I guess you could a noise suppressor on the noisy devices, but I haven't actually tried that yet.