I think they're completely sufficient, especially after Vogler's description.
Were the target anyone but Big O (and maybe also Fau), they would be sufficient. However, Roger is an extraordinary pilot and Big O an extraordinary megadeus, even though both of their capabilities are understated (two great aspects of the show, since you never knew what would happen next...). So, many then downplay Duo's missiles.
Fau doesn't even need a "Final Stage." What the chest cannons and tentacles lack in explosive power, which is to say they lack barely anything at all, they compensate for by being able to fire rapidly and in multiple directions.
I always took Fau's "rocket punch" as being a type of "Final Stage" (even though stills reveal it wasn't Fau's final manuever) because the chest cannons and tentacles can fire several rounds, but
you only have two fist's to launch, and as we saw, your fighting capacity (or at least your melee) capacity is diminished after that attack. That is, IF of course your opponent survives...