Well I didnt used the circuit the same circuit but have used the same principle to increase the amperage (only one transistor) of one of my circuit when I was in college and it just worked fine for me.
I guess this is a well Known method of increase the amperage for power supply
the only drawback would be that the regulator is a linear regulator. hence if you draw too much current and drop too much voltage across the regulator, you would be dissipating lot of heat. you can use a switching regulator instead (smps of an old comp.)