I'm sure you searched long and hard to find the best cut-off point as to what matches to include in slams to pretend it's OCC.
Nah, I've got better things to do. Finals and semi-finals were the first and only matches I looked at. It seemed reasonable for a couple of reasons 1. To consider all rounds would have involved over 2500 matches, in many cases involving players who I would have no idea if they used OCC or not. 2. You would expect finals and semis to be between reasonably evenly matched players.
Of course - if we included all matches in slams rather than just SF and F we get a worse figure because Serena, Venus, Justine and Kim did actually lose to other players in earlier rounds.
I dout it, not by much at any rate. Sure they lost a few but they also won a helluva lot of GS matches over that period. By only looking at finals and semis I was ignoring all the 1st, 2nd, 3rd and 4th round matches that they won over the same period.
However, in response to your point that the stat is indicative of nothing more than Serena, Justine , Kim & Venus being a helluva lot better than everyone else, I did the same for finals and semis of MPs and P5s since the 2008 roadmap. I've also adjusted the results for slam finals and semis to cover the same period (though the %age isn't much different if you include 2007). Oh, and before you ask why I've only included MPs and P5s and not smaller events, it's because the strength of the field at PMs and P5s makes for a reasonable comparison with slams.
PMs & P5s 2008-12
F&SF won by the player who used OCC - 7 (33%)
F&SF won by the player who did not use OCC - 14 (67%)
Grand slams 2008-12
F&SF won by the player who used OCC - 2 (9%)
F&SF won by the player who did not use OCC - 21 (91%)
So yes, the players who eschewed OCC were better players overall. But the players who used OCC were 3˝ times more successful in big matches against them when OCC was available than they were when it wasn't.