Certainly if the circuit's function is for just using as a programmable 'dummy load' then worrying about efficiency is silly, it's all going to be converted to heat anyway. However what you have drawn is really a programmable constant current source useful for continuous operation for say driving high power LEDs or other constant current applications where increased efficiency is always welcomed. I guess these days one could look at high current hall effect current shunt sensors, their costs are higher but they would result in the most efficient operation.
I recent completed building a 100+ watt oil cooled 50 ohm dummy load used in ham radio transmitter testing. I used 20 1,000 1% 2 watt resistors ordered on E-bay for just a couple of bucks. I wired them in a circular parallel manner and soldered them to a BNC connector mounted on the lid of a 1 quart paint can, filled the can with mineral oil and have been able to pump 100+ watts of RF into it for many mins at a time. After about tens mins the can is warm to the touch but handles the load nicely.
Lefty