To be a little more clear, you can't enter some of these events unless your PP (Performance Points) are below a limit. So you can add ballast (weight) or lower your horsepower or downforce, and thus reduce a car's PP.
You get better results by maxing your car and then adding ballast to get to the required PP* than by taking off your bolt-on go fast bits or just reducing power. (*If you can get the PP that low, that is. If you can't, add as much ballast as you can & THEN reduce power by the required amount.) I think it's a torque thing? I tested this extensively with the K Car seasonal.