KresoF1:

 

M5 is the best car overall. E63S close second. M5 is also very, very fast. In straight line faster then 991.2 GT3 or 991.2 GT3 RS. It can follow 991.2 Turbo S on the autobahn at high speed without any problem. Only 488 GTB and 720S are clearly faster on the autobahn. I am very impressed with new M5. Best car from BMW in last 10 years or so. 

This is an interesting video:

http://ausfahrt.tv/youtuber-sport-auto-0-300-km-h-bmw-m5-vs-amg-e-63-s-vs-porsche-panamera-turbo-s-vs-cadillac-cts-v-sport-auto/

Starting at around 0:40 you see the acceleration of M5, e63s, Panamera turbo s. It seems these three cars are nearly identical in terms of 0-300 - the M5 is a bit quicker than the Panamera turbo s up to 200kph. At highest speeds the Panamera seems to be a bit better than the M5. 

It seems (in this video) the tested cars do 0-300 in about 31-32s, i.e. very quick. A 991.2 turbo s might be more like 28-29s (in normal or sports mode, i.e. rear wing in normal position). Thus, the difference is not much (in a straight line).