An overboost would have been catastrophic for the mission.
JWST has thrusters on only one side, and to correct for an excess of velocity it would have had to rotate such that its instruments faced the sun, ruining them instantly.
As such, the mission design called for a deliberate underboost, to be made up for by JWST's own precise thrusters. But a more precise launch vehicle means that the error bars are smaller, and they can use a much smaller underboost than would be required from a less precise launch vehicle.
JWST has thrusters on only one side, and to correct for an excess of velocity it would have had to rotate such that its instruments faced the sun, ruining them instantly.
As such, the mission design called for a deliberate underboost, to be made up for by JWST's own precise thrusters. But a more precise launch vehicle means that the error bars are smaller, and they can use a much smaller underboost than would be required from a less precise launch vehicle.