Here is some theoretical justification of why I think the approximation may work. The resolution of the entire lens-sensor-camera system, (R system), is given by the empirical equation that is usually cited in textbooks:
1 / (R system)² = 1 / (R lens)² + 1 / (R sensor)² + 1 / (R low bypass filter)² + ...
where R lens is the resolution of the lens etc. Under conditions where the weak link in the chain is the resolution of the lens, then the equation approximates to:
1 / (R system)² = 1 / (R lens)², as all the other terms are relatively small.
That is: (R system) ~ (R lens)
The question is how do we measure (R lens)? Some people use MTF 05, others may use higher values. However, if we are comparing two lenses, lens1 and lens2, then the ratio of MTFs at other values, eg MTF 50, MTF 20 etc may be used as an approximation. So, even though we know that the resolution of the whole system, (R system), depends on the camera etc, as chromophore and others correctly say, under conditions where the resolution of the lens is the dominant factor:
(R system)lens1/(R system)lens2 ~ (MTF 50)lens1/(MTF 50)lens2
As seen from the data I calculated in the previous post, the calculations on MTFs obtained from a 5D FF and a 350D crop from Photozone.de were similar, and I have also found the ratios of MTFs measured by lenstip using a 20D or 1DSIII gave similar results to those from photozone. So, experiment shows it is not entirely stupid to make the assumptions.
As I keep emphasizing, I am not trying to provide a serious analytical theory. All I want to know is what is roughly the relative performance of different telephoto lenses. I know that my 100-400mm L is not nearly as good as my 300mm f/2.8 with a 1.4xTC. But, I also know that if I move closer to the subject with the 100-400mm, I can get an image that is as good as the 300+TC at a further distance. My equation gives me an idea of what that is worth in practice in terms of distance. It also tells you that you that the Sigma 150-500mm at 500mm is about the same as using the good old 300mm f/4L for photographing a distant bird, which I think is useful information, if correct.