Flotsam: Insanely Fast Floating-Point Number Serialization for Java, Javascript
blog.factual.com
blog.factual.com
I had good performance gains versus JSON as evidenced by this popular JSPref:
Regarding this specific approach, "only 20% overhead" sounds pretty good, but base 64 encoding has "only 33% overhead", is completely general, and likely already has faster implementations than this.