With no direct disrespect intended towards these authors in particular, I'll believe "expressivity of Scheme with the convenience of Python, Lua or Javascript and the performance of C" when I see it. Goals aren't results. I've seen that goal a lot. Given how often I've seen it, and how I'm yet to see a result that matches it, I'm coming around to the assumption that that set of goals is probably just not possible. (I suspect Python, Javascript, etc. are fundamentally, in their very core, based on some operations and conveniences that are just fundamentally going to be slow on modern processors. The only way around it is to drop some of the conveniences. That I have seen be successful.)

On the "goals aren't results" front, the second one is also one that I'd want to see how it plays out a bit more before getting too excited; it's also possible that it may not be possible at it currently exists in the author's heads.