I copied these draft-calculations of mine from another thread to underline the importance of the diffuser:
Downforce = Area * Density * (speed2^2 - speed1^2)/2.
Beginning with a 24000 N (2400 kg) total downforce at top-speed, 305 km/h (85 m/s), a squared relation between speed and force results in half of that downforce, 12000 N, at 215 km/h (60m/s).
For the sake of argument, a contribution from diffuser and floor of 20% means 2400 N at the same speed, 60 m/s, which with an estimated floor-area of 2 sq. meters in turn would suggest an air-speed under the car of 75 m/s.
Now, imagine that Brawn with their "innovative" diffuser, manages to increase underbody air-speed with 4%, from 75 to 78 m/s, downforce from diffuser and floor would jump from 2400 to 3000 N or 25%.
Overall downforce at 215 km/h, would as a consequence increase from 12000 to 12600 N or 5%.
On a curved section, everything else equal, a 5% increase in grip should in theory result in a 2.5% higher speed, when lateral force is m*v^2/R. With 40 seconds, or half the lap, spent in curves, it gives a 1.0 sec advantage over the whole lap.