No one is "ignoring available data". Dart's type system, in particular its covariant subtyping unsoundness, outraged Andreas Rossberg of the new V8 team. He expressed himself plainly about this on LtU the other year:
http://lambda-the-ultimate.org/node/4377#comment-67586
Andreas and many others among us TC39ers are well acquainted with Dart. I think this intra-Google, anti-Dart outrage motivated SoundScript, in part.
That doesn't mean the "data" is out there on some Dart public wiki to inform SoundScript's design in ways that programming language research and development do not already sufficiently inform it -- that we should just go get it, or we're slackers. I think you know this.
Yes, all dynamic language VMs will look more alike in 10+ years. StrongScript or a closely related descendent of it in 5 still beats Dart, and I'm still right that this took too long by 5 years at least, precisely because of the bad politics and JS-can't-be-fixed-but-it-can-be-replaced fallacies of the Dash memo that pre-forked and diverged Dart too much, without JS uplift apart from SIMD.
Are you done defending this sideshow as a productive experiment that helps the Web? Because so far, apart from John McCutchan's work, it has not helped. Maybe it will emerge a dark horse winner, but odds are against.