Actually, it's been known for quite awhile that there's a factor of 2 to 3 times in productivity. That's not really marginal. Talent far outstrips that, though.
It doesn't equal utility, but it does equal quality, at least to the extent that bugs per SLOC matter. This has also been known for awhile, with data to back it up.
I'm not sure that's true. Though it obviously depends more on the programmer themself, the semantics of a language can make it easier, at least, to write higher quality code. For example, bounds checking removes a whole class of bugs (probably the most common in C) from the equation. I disagree that this effect could be described as "marginal".