Giving the engine MORE down thrust will make the problem worse, think about it. The thrust angle is trying to pull what ever it is attached to in that direction, the more power you add, the stronger this force gets. If your aerodynamic forces are fighting the thrust angle you will be wasting power and turning what should be a fast plane into a speed sensitive mess. If the thrust line and C/L of the wing are in rough allignment, the engine should be pointed straight ahead [with maybe a touch of right thrust]. As a rule of thumb, high wing model designs with lifting airfoils need down thrust.