This would be the case if the grill guard is sealed around the grill. However since there is often a significant space between the guard and the grill that exceeds the amount of surface area of the grill by quite some margin air flow should not be a problem. Your assessment of the lights is spot on.Keep in mind that the more restrictive a grill guard is, the less air the fan can pull to cool the motor. That and now your headlights will be compromised as well. They will reflect off the inside of the grill guard and not work very well.