Right it's not an "officially supported" upgrade path, but I did it and it worked great. Upgrade install from RC (Ultimate x64) to RTM (Ultimate x64) using the cversion.ini mod published by Microsoft. As I said, worked like a charm. Been running since RTM was first leaked without any issues related to the upgrade.
But, that said, clean install is always the best approach. My plan was to try the upgrade and if I saw any problems, I would clean install. In other words, nothing to lose by trying it. But since it went so well, I saw no need to clean install although I was prepared to if necessary.