I've read that the jet boat's impeller system is about 30% less efficient
than a similarly powered outboard. Does that mean that a jet boat will get
beat by an outboard, or just use more gas?

I've got a Whaler Rage 14, and skipping along at 25-30mph it seems efficient
when lightly loaded.
It will go through 5 gallons of gas after several hours of mixed running,
but I haven't done any extended runs that would allow me to measure the mpg.