I see Sam and Capriati as completely different players, but the one thing that can be said for both of them is that they can be a really tough match up for Serena, they are tougher opponents for her than many other significant tennis players from this century, especially Capriati, who is a nightmare match up for Serena, although we all know that Serena is a much superior player.
I also think that peak Sam would be a very tough opponent for Kim. Although none of them played their best tennis in that match in USO 2010, it's clear that the surface is very good for Sam's game. It's a shame she couldn't beat Vika this year. She has the game to beat her, but I'm not sure that she truly believes in that. She's capable of hitting outside Vika's hitting zone, but she can't do it consistently... Or she doesn't believe that she can, the outcome is the same, however.
Capriati was superior to Serena on Clay but otherwise I agree with all you said.
And yes, I do agree that Sam and Capriati have much different gamestyles. The similarity though is that they (for different reasons)
are not good matchups for Serena.
And on the Vika match this year at the US open.... I really believe that proved how much heart and determination Stosur has. She was insanely close to beating Azarenka that day.... it really just came down to poor luck on Sam's part.
And on the respect of not being able to outhit Azarenka consistently.... I think that is simply mental... certaintly it is not because of her groundies. When Stosur is in the zone I would argue she hits a harder (though not as precise or consistent) ball off both wings than Azarenka does.
However when Sam is not hitting effectively (most importantly that BH wing)
Victoria can pick on the BH and break Sam's game apart that way.
As good as Vika is her game is effectively a glorified pusher. I think of her as a more competent version of Radwanska, Woz, etc.
She certaintly does not hit the ball as hard as Serena, Kvitova, Sam, Li, etc.