sure - i agree...
as you say with all the random elements involved it is almost impossible to quantify but Proficient GS players win matches - full stop...
on the flip side I dont see much difference between a mediocre/respectable do you think the drop off in performance is less severe at the lower end of the scale?
To tell you the truth, I am not sure. I so rarely play mediocre GS players in meaningful games and at meaningful positions that I have no view on this issue.
It doesn't make much sense though, since low levels or GS usually happen when you don't play the player enough (I am yet to see mediocre GS for a player who's getting 40-60 minutes week in and week out). Logically, the change should be linear, but of course this is just speculation.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."