OK, I can't come up with anything more quantitative, which I don't think anyone can. I think we can agree on the following points. For high class telephoto lenses like the series II big whites, the resolution is pretty close to being limited by the sensor, filters etc. For that reason, the measured MTFs on cameras with different sensors are close to to 1 (as I wrote previously, 0.978 is good enough for me).
When you get MTFs that become significantly less than 1, the lens itself is the weak link in the whole resolution system and its MTF largely determines the overall measured MTF. For a lens like the Sigma 150-500, which has a measured MTF of 0.52 at f/6.3 and 500mm, the resolution of the system is pretty close to that of the lens itself.
So, I would say to a first, very rough, approximation, you would have to get twice as close to the subject to make it 2x wider and 2x higher in order to get an image to reveal as much detail as a very good 250mm lens at the same distance. So, I would equate that to the Sigma having an effective focal length of ~250mm for telephoto purposes.
The 400mm f/5.6 L lens is described by many of its devotees as being very sharp. It is not tack sharp, but if you get closer to the subject than you would have to get close with a tack sharp lens, like the 400mm f/2.8, you will get an apparently very sharp image because the image has a larger size and covers more of the sensor. I estimate in practice that it performs as about as well as a 330mm tack sharp lens. But, of course, there are other factors that come in as well.
I'll leave it at this point and thank everyone who joined in the discussion.