Yes, the rudder boost assists in the objective of keeping below the 150lb, but the root cause is the asymmetry. As the Kingair increased in size over the years and got more powerful engines the asymmetric forces increased at OEI T/O. It wasn't fitted to the early KingAirs with smaller engines, but on later models / variants it's mandatory. It may also be that the aircraft actually just runs out of rudder without AFX. That's a Beech test pilot question.