If you choose to perceive sarcasm and humour as some sort of personal insult, then the internet probably isn't for you, perhaps try a subscription to Woman's Weekly or something instead?
Powerline products usually quote speeds based on symmetrical throughout or full duplex, e.g. a normal 100mbit wired connection in the world of powerline is quoted as 200mbit, it's not technically a lie as in theory hosts could send and recieve data at the same time up to the maximum link speed in both directions, but it's not the way in which connection speeds are quoted for other networking products - it'd be like me quoting my FTTC 52/10 profile as a 62mbit connection. Often you'll find powerline products such as the AV500's etc. are supplied with 100mbit ports, good luck getting 500mbit of data through a controller that's only capable of 100mbit. In your case you have gigabit ports, obviously 1200mbit is not going to happen via a port that can only support 1000mbit in any one direction, so that's 600mbit either way max, your 505 sync likely translates to 252.5mbit either way (from a kit that's capable of 600mbit each way).
So that brings us to my sarcasm which you clearly didn't appreciate or understand. You posted your link speed, as we know, that's not really the speed you can send data from one local host to another at, but then you shot your self in the foot by suggesting that your ability to test the link speed between two powerline adapters on your LAN was in some way dependant on your WAN speed, thats a bit like going to a race track and saying you can't go faster than 30mph because that's the last speed limit sign you spotted before you came in. That's why I pointed out your credibility just went down in flames. I kind of hoped you'd realise and get the joke without having to resort to a wall of text, but hey it filled in 5 minutes.
For the record, I actually like Powerline products for certain scenario's, but the key to using them is to understand what they are, how to use them and thetevhnical limitations.