Direct3D only really exists on Windows (and maybe on their XBOX, i don't know), so you can't say that it is faster on Windows. Also, the Graphics-Driver handles that. As far as i heard, AMD-Cards/Drivers tend to be better at Direct3D but it isn't stopping you from using OpenGL.
Have you looked in the settings in the Direct3D-Backend? Things like EFB or XFB? Did you tried to set the settings like in the OpenGL-Backend?
The blackscreen with OpenGL with the Steam Overlay might be a Steam-bug/something like that because the Steam Overlay is supposed to be "invisible" to the game. Also, you can take Sceenshots in Dolphin (I think F9 while ingame?) Also they are saved in PNGs, wich are better in quality than jpeg and smaller than TGAs (wich Steam uses for uncompressed images).)
Have you looked in the settings in the Direct3D-Backend? Things like EFB or XFB? Did you tried to set the settings like in the OpenGL-Backend?
The blackscreen with OpenGL with the Steam Overlay might be a Steam-bug/something like that because the Steam Overlay is supposed to be "invisible" to the game. Also, you can take Sceenshots in Dolphin (I think F9 while ingame?) Also they are saved in PNGs, wich are better in quality than jpeg and smaller than TGAs (wich Steam uses for uncompressed images).)