Folks,
I am not specifically familiar with the NG systems so I will presume that Finalveridict is correct in saying that "According to the AFM, the WTAI is supposed to come on when you have MCT on the good engine".
The selection of MCT normally occurs at the start of the 4th segment and is an acceptable power reduction in accordance with FAR 25.121 (c)(1). Provided that there are no 4th segment obstacles to consider and the minimum certification climb performance requirement is met, there is no practical or legal problem and the references to the TOGA thrust limit and acceleration altitude are quite irrelevant.
The AFM data will allow you to calculate a TOW such that the first three segments are completed within the TOGA thrust limit using whatever acceleration altitude the operator nominates. As that acceleration altitude gets further away from 400 ft, TOW often must be reduced to allow the extended 2nd segment and 3rd segment acceleration to be completed. With high thrust engines in icing conditions, an acceleration altitude of 1500 ft may not require any TOW reduction at all - the answer will be in front of you in the TOW data.
I suggest that you read all of the relevant provisions, including subpart G and FAR 25.1533 to appreciate that 400 ft is the minimum, but not the only, height to demonstrate compliance with the take-off flight path provisions.