After thinking about this for a while I'm not as enthusiastic as I once was about adding mass to powered planes for penetration. Gliders are a different story. In an uplift, the airstream (and by definition, drag) has a small vertical component that can be countered with increased weight. Without a propeller any small increase in forward acceleration is important. Looking at it graphically using force vectors it's apparent how adding mass improves "penetration" by offsetting the drag.

When we apply down elevator to add forward speed it's the same thing. We are pointing some of the weight vector in a direction opposite the drag.