Looking at the 720p films that I have, all but one has 5 reference frames. These are all 'scene releases' downloaded via bittorrent hence a very popular if unnofficial 'standard' on the internet.
If you could push the capabilities up to 5 reference frames then I think a lot of cheapskates would be very happy. In the meantime at least now I know what to avoid downloading.
Many thanks to all of you that are working on this, it is very much appreciated