It is not just the fatigue factor as Jad pointed out; it is also a question of safety. A point more germane for you as a racer, is that in event of radical decleration (i.e. shunt, wall, etc.) the weight of the head and hemet are combine and are multiplied 30-40 times (depending on the speed of the car) so every ounce of hemet weight applies X multiple of force against your neck. This can exceed 1,300 pounds of total neck load.
Anyway, without delving further into the specifics, just know that when it comes to hemets, lighter is safer.