Lucas said that Anakin would have become twice as powerful as Sidious if he'd reached his full potential, but of course, he went on to change that, and it was an off-hand comment. It shouldn't be considered an accurate statistic . It's just a good indication that Lucas wanted Anakin to be far, far more powerful than the most powerful sith lord in history. Same goes for Vader being 80% of Sidious' power. Just a random statistic he thought of which shows what he thought of Vader and Sidious, but shoudn't be taken as an actual statistic.
His interpretations of the characters of Maul/Dooku/Vader are also good ideas of what the characters are meant to be, but there's just too much content out there to rely solely on Lucas' comments. What we interpret from what we see takes precedent over Lucas' interpretation.
Now, legally, some comments are considered official, but they mean nothing to me, since authorial intent is just meaningless when a work is open to interpretation. I know you're going to say that we're on an official site and we need a standard of what's considered valid, but the way some of our members here, especially Az, twist the story to make their desires for the universe come true (but support it with textual evidence), it's basically as good as being open to interpretation.