can't speak of the nose-low and firing deal, but when a plane is in level flight if firing the smoke would be above the wings as the guns are not actually firing directly ahead, but are pointing slightly up so that the fired rounds' trajectories arc up then back down to a point ahead of the plane, based on where the convergeance is set. Just like when standing on level ground firing a rifle, your sights compensate for distance to target by causing you to elevate the barrel ever so slightly so that your round "falls onto" the target.
As such, as the rounds are fired from your cartoon plane, they actually climb above the flight path of the plane, so the smoke trail is above the wings...
