I conducted some simple benchmarks using iperf via jperf on my pair of Macs: a 13” 2009 Macbook Pro and a 13” 2012 Macbook Air. Each has gigabit Ethernet, though the latter uses a Thunderbolt adapter for it. I used also the Rosewill T600N wireless router as the switch device, primarily to dole out IP addresses when devices are connected. All devices reported gigabit Ethernet connections to the adapters.
I conducted 15 tests on the Rosewill units and a few of the same tests on a pair of TRENDnet TPL-402E adapters I acquired last year. They’re largely the same specification, but have a smaller frequency band (50 MHz max instead of the Rosewill’s 67.5 MHz) and suck one watt additional juice under load. The TRENDnet adapter kit also has a “bonus outlet” on the front, so you don’t lose an outlet to your Powerline system.
Here are the results of my testing. The background color in the average throughput column indicates the link quality reported by the remote unit’s data LED. Raw results here.
You can see that, per norm, the farther away you are, the lower throughput is.
Basically, one the same circuit, throughput is 80-100 Mbps. On a different circuit, it’s going to be anywhere from 20 Mbps to 40 Mbps. Increase the wiring length and the throughput drops quickly. Judging from where in my house the adapters were located for the last two tests, I’d say that the wire distance is likely pushing the 300 meter length limit between the two adapters.