In the video the M3 takes the race but the timeslips show the Mustang was quicker and slightly faster (reaction time is a large factor in a race). The M3 with its smaller motor is able to take the race thanks to its quicker shifting transmission and higher redline. The owner of the M3 switched his exhaust setup which is costing him about a mile per hour of trap speed so we can call this one a draw. Pretty interesting considering the smaller motor and non-intercooled supercharger running low boost.
The video makes it evident the M3 is making more efficient use of its power. No surprise thanks to the higher redline allowing shorter gearing and the transmission providing much quicker shifts.