The boeing 400' AGL is for take off. The GA requires 2.5% min (sometimes more if written on the chart) If you look in the performance section of the ops manual there is a chart with pressure altitude and weight which will give you the expected SE GAclimb gradient. If you cannot achieve 2.5% you will need either a visual escape manoeuvre or a higher minima. If the climb gradient chart gives you higher than 2.5% you know that you can fly the standard missed approach HOWEVER if you accelerate before MSA or missed appr alt you now do not know what the average climb gradient has been so you are no longer safe. My answer 9sorry its been a bit dragged out) is on a single engine Ga you must not accelerate until you reach MSA or missed appr alt. Hope that helps