There are a variety of synonyms for the phrase "went flat out" that convey a sense of extreme effort or maximum speed. Some possible options include "raced," "sprinted," "hurried," "hastened," "rushed," "galloped," "charged," "dashed," "bolstered," and "pushed oneself." Each of these phrases has a slightly different connotation, with some emphasizing urgency or haste and others highlighting the endurance or exertion of the effort. Regardless of the specific synonym used, they all suggest an all-out effort or a maximum use of energy and resources to achieve a goal or complete a task.