I think it would be more interesting to compare VP9 with HEVC (h.265), I know the VP9 experimental branch can easily be checked out and tested, but I don't know of where to find HEVC encoders (although I'm sure they exist with some Google fu).
Obviously there are problems with testing unfinished implementations, as:
A) they are generally VERY slow given that there's little optimization during the development phase, and these next gen video codecs are more demanding than the previous generation
B) a great deal of the quality comes from fine-tuning during implementation, not from the specification. For example x264 is a 'best-in-class' implementation of h.264, readily beating many other implementations of the very same specification.
As such, this fine-tuning is not likely to exist until the codec implementations mature, which likely means that what is available now of HEVC/VP9 serves only as very rough estimates.
Obviously there are problems with testing unfinished implementations, as:
A) they are generally VERY slow given that there's little optimization during the development phase, and these next gen video codecs are more demanding than the previous generation
B) a great deal of the quality comes from fine-tuning during implementation, not from the specification. For example x264 is a 'best-in-class' implementation of h.264, readily beating many other implementations of the very same specification.
As such, this fine-tuning is not likely to exist until the codec implementations mature, which likely means that what is available now of HEVC/VP9 serves only as very rough estimates.
Would still interesting though :)