Improving efficiency doesn't address the problem at all. If you only improve efficiency then consumption grows to negate any gains.
But, what I think you're saying is that if we can increase efficiency by X%, then we can still sustain current economic output, yes? Well, considering we're 70% over the sustainable level, that means we'd need to get 58% more efficient, with no increase in consumption, in less than 20 years.
How likely do you think that is?
Our current energy needs could be harvested from the sun exclusively. Cover sahara in solar collection and you satisfying energy requirements of all of humanity, several times over.