Depends. If you've got near perfect air superiority, it is. If you need to worry about your planes being intercepted, being faster can sometimes help quite a bit. It can also help with time to target area. For the US at the time, speed was not a prerequisite of the competition. New built orion like airframes were competing against p8 proposal. Its possible boeing won , besides political clout, because small run of orion like aiframes was fairly expensive, while p8 could rely on sheer volume of b737 production to eek out a competitive price.