Efficiently limit current to led without power loss
The circuit you have indicated does not lower the power loss. It merely replaces your fixed resistor with a current limiter that ensures you have closer to the 300mA or whatever you need, under a wide range of LED forward voltages and supply voltages.

As such the current limiter is really just a smart resistor and will still dissipate the same sort of heat a simple limiting resister would.

To be low power you need a circuit that uses some sort of switch mode regulation to generate the required current in the LEDs at a high efficiency conversion factor.

UPDATE

By the way your math is wrong.

You stated..

Power loss through resistor \$= 7*0.3=2.1W\$

That is incorrect, that is the formula for voltage drop across resistor = \$2.1V\$

Power loss through resistor \$= 2.1 * 0.3 =0.63W\$

You are already running at 82% efficiency. 

With a switch mode current regulator you might be able to boost that up a few percentile, but it may not be worth it. It is still wise to use a current limiter rather than relying on a resistor though.