Incorrect.

The linear drivers are NOT efficient. They take the extra voltage and turn it into heat.

The DC boost diodes however are very efficient. Atleast 80%. Sometimes over 90%.
Efficiency is calculated by

(Watts out / Watts in) x 100 = % efficiency

For example: a linear driver like rkcstr.

Takes in 7.25v at 100ma. So 0.725W in. (btw that's the minimum V needed for bluray)

Outputs about 5V at around same 100ma. So 0.5W

(0.5 / 0.725) x 100 = 69% efficiency

If input voltage is 10V then, input would be 1W. So..

(0.5 / 1) x 100 = 50%

So basically, the best efficiency a linear driver gets for a bluray is 69%.

Then the efficiency drops as voltage is increased since the extra power is converted into heat. This is why linear drivers get hot.
The DC boost drivers however DO NOT CONVERT EXTRA POWER TO HEAT.

This is because they are given lower voltage than output voltage.
So say a flexdrive has an input of 3V and needs to output 5V @ 100ma to drive a PHR.

That means it outputs 0.5W (5 x 0.1).

3v x (input current) = 0.5W, therefore input current = W / Vin

So input current will be about 167ma.

However since they are about 90% efficient. The Input current will actually be about 185ma.

So in this case at a efficiency of 90%, the boost driver takes in 3v@185ma and outputs 5V @ 100ma

Hope this made sence

Feel free to point out any errors.