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When you activate a displacement map in ZBrush, by default it is simply a bump effect. This means that no matter how far you crank up the intensity, it will not alter the silhouette of the mesh. In other words, it will simply not give you an accurate preview of what is going to happen to the mesh when you apply it to the actual geometry, at which point the amount of displacement will be determined by your map, by the intensity setting, and by the midpoint value.
To get a somewhat more accurate preview, you must enable the Mode button. Though this isn’t 100% accurate either, it will at actually alter the silhouette so you can get an idea of what your intensity settings are doing.
As far as the artifacts around the nose, this probably has to do with UV strategy. Keep in mind there will always be some sort of visual issue where there are visible seams, which is why you want to lay out your UVs in such a fashion that the seams do not fall in high scrutiny areas–like the face.Unwrap your mesh so that seams occur along natural breaks in the surface, or tucked away in low scrutiny areas.