Okay I've attached an edited picture where I drew a red line around where I believe the "effective area" of the vents on the P1 is. I might be drawing it too big, they might have designed the edges of the vents in some tricky way.
Yes it's still huge, I'd say maybe about the same size as the one on the hood of the Corvette or the Evo. But look at a picture of the front of the P1.
The intake for that radiator is half the height of the nose, and extends nearly all the way to the headlights. My eyeball guesstimate is that the vent is maybe 30-40% bigger than the intake. Plus, the nose is raked down sharper than the front engined Vette or Evo, so the pressure there is higher in the first place.
The Evo has a puny intake that's half the size of that. There is no way enough air is making it to the other side of the radiator to be reducing lift. I bet the air in that vent is moving too slowly and is causing lift and drag.