F40 is only quicker in the low-end range because of the turbos and the longer gear ratios that are compatible with its torque curve. The actual winner here is the F50. It doesn't launch as good or accelerate as hard solely because it is a naturally aspirated high-revving car with identical peak power and peak torque.
The F40 makes 522hp@7775rpm (actual) while the F50 makes 603hp@8700rpm (actual). Their power-to-weight ratios are 379hp/tonne and 408hp/tonne, respectively.
F40: Zero to 30mph in 2.0s, 60mph in 3.5s, 100mph in 6.9s, 124mph in 11.1s, 155mph in 17.9s, 186mph in 29.3s, and 200mph in 47.9s.
F50: Zero to 30mph in 2.1s, 60mph in 3.7s, 100mph in 7.1s, 124mph in 11.4s, 155mph in 18.3s, 186mph in 27.6s, and 200mph in 33.6s.
Honestly though, I view the F40-to-F50 transition as similar to a LaFerrari-from-SF90 transition.
Also, the F40 makes 511lb-ft@4750rpm whilst the F50 makes 364lb-ft@8700rpm.