Raph really, really knows his stuff. His interview on the New Rustacean podcast starts with a summary of his background: Gimp, Ghostview, android font rendering.
Your 10x speed, 90% functionality caution is definitely warranted. I've lost count of how many times I've seen "we run Python/Ruby/Perl/whatever" 3x faster, just haven't implemented exceptions and monkey-patching yet. :-)
Yup, I still remember being blown away by his dissertation. That he is using Rust in earnest is enough reason for me to take another look at the language.
For what it's worth; Gimp, Ghostscript, and Android all use FreeType for text rendering. At best, they would require him to know how to use FreeType bitmaps correctly (i.e. gamma-correct blending, when to multiply colours in). Though he could easily go beyond that, it is very interesting stuff and I find it incredibly hard to stop myself from reading into FreeType.