Pretty sure I once ran the converter (they are totally passive with no odd conversion taking place so it's all down to build quality) probably from Maplin years ago when I ran analogue into my
SGI 540 via a Sony broadcast video matrix. It was pretty good (broadcast quality) for SD displays, but not sure what upsampling onto large screen would look like - but same goes for S-Video.
Well done, composite and S-Video is pretty close, but compared with uncompressed native HD, it's not going to be comparable. But that's the limitation imposed by F80 and you can only work with what you have and try not to degrade signal.