If you decrease downforce enough they cannot sustain Gs through a turn because they cannot take the turn as fast due to reduced lateral grip regardless of how much power they have avaialble. It is a function of speed, tire grip, turn angle and banking angle. You cannot sustain 5 Gs through a turn (even banked) in a car without downforce. If someone tries to draft, they will lose even more downforce. This lets them go even faster on the straights but they will have to brake for the turns or they'll go into the wall. If the only problem is G forces sustained in a turn, this would be a good solution. If they are concerned about top speed down the straights, it would not be.
Adding a bunch more downforce is certainly another way to drop speeds (on the straights as well as in the turns) way to decrease speed is to add a huge amount of downforce which creates more drag. But it is is not as precise for limiting speed through the turns because people can build more speed down the straights by drafting, and since they are set up for more downforce than they need, they can carry that extra speed through the turn. Same for minor adjustments to boost, etc.
[This message has been edited by Lud (edited 04 May 2001).]