It may help those who are still not sure about the effect of weight when in a dive to consider the simplest case of a glider that is being held in a vertical dive.
Here only two forces are in play, the weight acting downwards and along the flight path and drag acting upwards and also along the flight path. When the two are in balance (assuming the wings stay on) the glider is diving as fast as possible. If the pilot now jettisons the water ballast he carries (in order to improve his performance in the type of competitions where speed matters) the drag will now exceed the weight and the glider will necessarily slow down until the two forces are again in balance.
If we add a motor to our glider in this dive the only difference is that there will be an additional thrust force in the same direction as the weight. The combined thrust and weight clearly will require a higher speed to generate the extra drag to again bring the upwards and downwards forces into balance. The effect of any weight change remains the same as with the glider case.