Steam survey doesn't show percentage of usage per OS for cross platform games. At least I've never seen such data published. It's tossed around often, but it's not useful really.

That's true. However, I'd wager that the data would still correlate with the hwsurvey results. Would be interesting to see the actual numbers, especially for CS:GO and Dota 2.

I don't think it would. I saw some reports from actual developers, that differ quite a lot from that survey. So, I wouldn't use it as any sort of definitive metric.