Anyone knows whether the mesh optimization functions in the D3DX library do their work based on the device in use, taking into consideration hardware properties such as vertex cache size and the likes, or if the optimization will be identical on any hardware?

OPtimize will take into account current hardware if is recoginized, othwerwise it will default to a setting that is pretty good for most hardware. There is probablly only a 3-5% possible difference from the default to the tweaked settings.