Power/weight makes a difference on acceleration, but of course has no impact
on cornering or braking. In any event, for an extra $15,000 - $20,000 would not one expect an ultimate driving machine to be superior in both weight and power?
Based on power/weight, the V-6 Mustang and F30 are closely matched:
Mustang, 305HP and 3523 pounds.
F30 300HP and 3591 pounds.
However, assuming the commonly accepted 320 HP for the F30, the F30 enjoys a 11.22 lb/HP advantage over the Mustang's 11.55. The BMW also has a torque advantage, as you point out.
By this measure, the BMW should win - but instead gets clobbered.
Motor Trend has the V6 Mustang at 0-60 mph in 5.3 seconds, quarter mile in 13.9 seconds at 100.1 mph. (The best I have seen is 5.1 0-60)
The F30 is significantly faster in a straight line, it hits 60 mph in 4.8 seconds and the quarter mile in 13.4 seconds at 103.7 mph. (the best I have seen is 4.6 0-60, but lots more in the 5.0 range)
This is embarrassing. The F30 is quicker in a straight line, but is being beaten in the corners
by the Ford.