GR follows the incentives of points, so that we see everyone playing the same "game". So, in PPSC, draw size matters, unless there's a solo, then centre count matters. And in WTA, draw size also matters- unless there's a solo, then the soloing player gets all the points.
I'm starting to think it would be worth adding a centre count scoring system too (where centre count matters in a draw, and if there's a solo, then the soloist takes all the points). Last time I mentioned this, someone pointed out that three scoring systems might be too many- perhaps replacing PPSC with such a system would be a good idea? Certainly I think many new players think that PPSC draws are by centre count.
To the OP- I don't think there's consensus on how to measure diplomacy players. I personally prefer consistency over Elo based systems like Ghost Rating. Does this person consistently get good results (draws or wins, and not survives or defeats).
On this site, draw size matters, and so I've wondered whether the stats page should also break down draw size. However, it's probably busy enough as it is.