Wow, answers by Belini and Roberto Scandura in persons, I feel lucky
Anyway instead of whining here I tried by myself: I put a 10 in qualify/race perf to Raikkonen and Hamilton, and a 10 in qualify/race perf to Ferrari. Here are the results:
*If you saved your season between 2 races, the game will use the updated performances = they are not stored in the savegame!
*If you saved your season during a race, the game will use the performances stored in the savegame
*A driver with a race perf of 10 can drive suprisingly well. Under the rain, Hamilton managed to spin off right from the starting line, but he managed to recover and quickly overtook the Super Aguri in front of him