It would depend on the vented bonnet but if properly done they should reduce lift which our cars are adversely affected by.
But not really down force as such.
For instance, if you were to shove one of our cars in a wind tunnel equipped with load cells (weight scales) under the wheels.
You'd notice the load on the front wheels would get lighter as the wind speed increased (lift) this is because air passing through the radiator grill has pretty well nowhere else to go than being pushed under the car, there by lifting the front of the car.
Now if you were to change the bonnet for a properly designed vented version the air coming through the radiator can now pass up through the vents in the bonnet instead of going under the car, thereby reducing lift, fitting a splitter will further improve this.
So in the wind tunnel, our car would probably have little difference in wheel weight as the wind speed increases, maybe even a small increase in wheel weight which equals downforce.
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The vents need to be in the low pressure region towards the leading edge (front) of the bonnet.
The vent openings themselves would need to be facing the rear.
The sheet metal around the spare wheel well will need modification to allow air passing through the radiator to get to the bonnet vents or there would be no point in the vented bonnet!
Fitting a front splitter alone should help reduce lift, it helps preventing air getting under the car before the grille.
It all depends how much ya wanna spend and how much work ya do!
I'm not aware of any vented bonnets on the market at this time, I could be interested in any that are.
Bang for buck the spitter wins
"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"
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