On the Richards benchmark it's 1.9x. After 2 years this is pretty impressive.
Due the nature of the Dart language the DartVM will make a number of optimizations possible that are not even possible in V8. So it will be a better platform for continued performance improvements. Time will tell...
To be honest, even if it was only doing 0.5x, I'd still use it because the tooling makes me more productive.