CSV is better at killing mobs than Beam Fire At Will.
It adds on damage, does not appear to have a damage cap, does not become less effective if additional enemies are in the targeted area, has better control over how that damage is dealt, AND effectively gives a pseudo-increase in the firing arc.
Even if it does perform better with a Gravity Well, the base performance is such that it's not actually needed for it to be effective.
The reason scatter volley makes FAW obsolete has nothing to do with its duration and everything to do with the facts that
1.) CSV is very directional and selective with targets = more damage per target. FAW shoots at everything within, at minimum, twice the radius and at maximum a complete sphere.
2.) CSV doesn't castrate your power levels and reduce your weapons to the status of nerf guns. FAW does.
Lowering the duration is how I think it should be addressed: flip it so that CSV is more of a burst damage against multiple targets. Currently, I feel that it impinges too much into how Fire At Will works, and is just one of the factors rendering beam weapons rather redundant against cannons.