That's difficult to say without benchmarking. Yes, you can offload some computations to GPU (you certainly are not going to parse the bitstream or VLD decode using shader programs), but do you get net gain, when you are shuffling buffers over PCI-E? You also limit your target audience to NVidia 9400 and up when talking about OSX, basically telling the Macbook users with Intel graphic cards, that they do not need to apply (Intel 3100 and 4500 are perfectly capable of decoding H.264 and VC-1, just not through shaders).