Will X2 really need DirectX10-11? I'm currently with DirectX9.0c... I mean, I was always told to upgrade or rise the requirements only if they are useful enough (and used).
Problem with Steam and Linux seems to be that most distros will only package what they consider stable applications as a whole, whatever the game you want to play on. The priority given to a gaming platform may also be relatively low. Hence, the result could still be suboptimal even if Goldhawk gave Steam all the balls. But it would be certainly better than doing nothing, though. I would advise Goldhawk to contact the PlayOnLInux team instead, or as well. I believe they know how to harness Wine when properly informed on games' functionality and API usages, and they are as user-friendly as Steam.