Using Netcomm Homeplug AV "200Mb/s" I get 50-60Mb/s throughput (i.e. ~5-6MB/s) doing file transfers between a Windows 7 machine running a linux VM and a Linux machine which was a good enough upgrade from a bridged 2.4GHz 802.11g where I'd see about 20-25Mb/s or 2-2.5 MB/s. So about a quarter of the rated speed. I didn't see much improvement when both were plugged into the same powerboard so that's probably about the limit.

So based on those numbers if wireless worked you'd see about the same thing throughput wise.

What you're saying about throughput I doubt though. The signals are going at what about 1/3 the speed of light? Distance mainly affects latency not throughput. Do a ping test. On a LAN you should get ~1ms (or better) pings if working well enough. Maybe that's not what you mean but that's how I read it. Maybe you did mean that the longer length would cause more signal loss/slower speeds.