Remember that OpenGL will use mipmaps properly and that D3D doesn't. That means that the actual image may be different, not just settings. In the detail options screen make sure that "3D Hardware Textures" is set to the maximum, otherwise it will not make full use of your mipmap chain. If you have a texture map which is 512x512, with mipmaps down to 1x1, then D3D will always use 512x512 as the first layer. OpenGL, if you have "3D Hardware Textures" on dot 4 rather than dot 5, will use 256x256 as the largest map. So OpenGL actually makes use of your detail screen settings while D3D does not, and that messes some people up when they aren't aware of this and try to compare quality.
Also, you need to use an Anisotropic Filter setting of at least 2x, since 1x is actually off. The 5.5 Launcher (not publicly available yet) fixes this to actually give you the off setting rather than tell you it's 1x.