I won't try to do the math but the resistor is as stated, a device that converts electricity into heat so it is a drain on efficiency if efficiency is based on lumens/watt.
Let me use a lame analogy here. Lets say you have a big commercial vehicle with a turbo-charged diesel engine that is state of the art in terms of fuel efficiency and it gets 8 mpg. You also have an old beat up moped with a shot engine that only gets 30 mpg. If you just need to get yourself to the store and back then the moped will suffice and it will get you there and back better than 3 times more on the same gas expense as the truck would. In this example, the moped is a more efficient solution based on mpg but it is much less efficient in terms of energy converted to power of propulsion.
Although the power efficiency of a resistored two stage light may be relatively poor due to the resistor, the run time increase can be so significant that the gain in light hours makes it a much more efficient device having the two stage option. If you can increase your run time by say 20X with a useable level of light, how significant is the efficiency of power conversion from watts of electricity to lumens of light? A two stage constant current device might give you a 50X increase so it is certainly more efficient and even desireable if available as an option.
With most flashlights and in most applications, I suspect that the real loss in efficiency is at the very end of the trail. Simply put, you generate light at some level of efficacy only to have it wasted as it is not needed or even on target. We spill more light than we need or use, in most cases. If you are using a light that is outputting 80 lumens but you only need or are only getting 4 lumens on target, then the waste there makes a resistor insignificant in comparison.
Don't get me wrong here, I prefer a device that excels in efficiency at all levels of energy conversion and delivery but I also realize that the photons needed and delivered on site is the most important consideration in overall efficiency in task accomplishment.
Efficiency can be defined and measured using different aspects of a system and the level of efficiency will vary as defined.