Your GPU is of a generation that is not even sold in any Apple product. So you can't draw any conclusion. Even if you think nVidia supports it fully, you can't know all the details. There's been many reports about issues with flashed or non-flashed graphics cards (even some that belong to GPU families that were sold in official Apple products), having problem in very specific areas like flash videos in safari, not working in certain games...Nvidia has actively been developing OSX drivers lately and according to the tech sheets any GPU that supports Open GL 4.0 has full support for Metal. However...
Metal isn't being used by the Finder (Netkas has tested this), it's still OpenGL based Quartz rendering. Preview app isn't going to be deemed important enough for Metal either and will still being using Quartz.
The issue I'm seeing with volumes of large files and Finder slowness has more to do with the system's I/O and file handling operations than graphics acceleration.