Congrat's on the build Matthias.
I am curious about your description of your flight. First off , it is not normal for it to drop during forward flight at full throttle.
Is your quad "dropping" because the motors power, or is the front of the quad pushing downward?
If it is motor power problems, the ESC cal that Tony mentioned could cure that, if it is not a battery power problem, i.e. low 'C' rating.
If your description of full throttle, (left stick full forward) is the same as full pitch, (right stick full forward), your quad is being flown into the ground because of too much forward pitch.