simply cut liquid fuel use to 1/4 of present levels
Life is a b....
A "sustainable" energy economy, no matter the technology, can not be sustainable adopted worldwide if it requires imports, since there is no place to import from if adopted worldwide. So "without counting imports" is a necessary (though not sufficient) condition for sustainability. I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.
In any event, if the only way to reasonably ... (none / 0) ... exceed 5% of current liquid fuel consumption is to import, that means that 5% is the highest reasonable target