On my PC, the FPS cost of going from low to high rendering settings with SMP4 disabled is more than the cost of rendering multiple overcast layers to the horizon with SMP4. Showing a framerate under "common use" is showing the FPS impact of the rendering settings, aircraft, etc. much more than the FPS impact of SMP4. Everyone has different aircraft, scenery, settings, and hardware, so that really wouldn't be useful. What I'm offering is a baseline.
I'm *not* saying "you will get 70 FPS with SMP4 under worst case weather conditions with SMP's settings maxed out." Just that it is achievable. Everyone's setup is different and it's impossible to make claims like that.