The hood vents and adding/aiding to under hood venting is to stop heatsoak of the air.
It may help engine cooling on extremely hot days, but it's main purpose is to stop heatsoak and give cooler intake air charge temps.
To answer you second question.
Yes an engine (engine temps / not intake charge temps) can run too cool for optimum efficiency & performance.
It would be pretty hard to do such in hot weather, but fairly easy to do so in cold weather.
