How do I increase (multiply) the value of a current sink?
Your circuit has another issue, if you were to try to build more than one of them you would find that the LED current would vary dramatically from unit to unit.  

That's because the beta of a 2N3906 (or most transistors for that matter) varies widely.

Your requirements are somewhat contradictory.  To get efficiency you need a switching current regulator, which necessarily has a controller, an inductor and at least a few other parts.

One example is here: http://www.ti.com/lit/ds/symlink/lm3434.pdf 

You can find other solutions (and depending on how much current you really want some more highly integrated solutions) by searching the TI website or many other semiconductor manufacturers.