s7 will never come close to m6 grand coupe. m5 much nicer than ugly panamera and s8 should be compared to m5 in which it loses. so i disagree with your assessment. the key will be the new M3/M4 becasue any improvement in performance over the current will just solidify it being the leader because the current M3 easily defeats the rs5
The S6/S7 are competitors for the BMW 550xi/650xi, not the M6. It will be the RS6/RS7 that are competitors for the BMW M cars and Mercedes AMG cars.
Porsche is just a different animal, and I have to say that no BMW, Mercedes or Audi compares. The driving experience is CLEARLY better than any of the other three, but you pay for it. Looks are subjective.
The M3 is a more dynamic car than the RS5, but it is doesn't "easily defeat" the RS5. In the hands of many drivers, the RS5 would yield a better track time due to the fact that AWD cars can be driven harder with much more control. The M3 just feels better and more "alive".
Honestly though, at this level of automobile, we are splitting hairs. They are all good.