I had an '86 Rx7. The shape and size of the hood vent played a large role in the cars aerodynamics. I bought a few spare hoods and made different designed vents. I did come across a design that actually helped front end stability, but it was through a fair amount of trial and error. I'll briefly mention my designs and results.
Design 1: Evo style open vented hood w/ no ducting.
Results: Front end lift at freeway speeds, and much lower under-hood temps while at low speeds.
Design 2: Traditional drop vent hood w/ no upper air guide.
Results: More front end lift at freeway speeds due to a larger low pressure pocket of air. Engine bay temps slightly greater than above design with greater stability in coolant temps.
Design 3: Traditional drop vent hood w/ a full duct and smaller exit than entrance.
Results: Front end was more stable than above designs and felt superior to an un-vented hood. Much more stable coolant temps under load.
First 2 pictures are of Design 1, and show the space available.