On normal condition it needed 11.30~ second-ish for vengeance to emptying the clip, while with fire storm it needed around 10.30~ second-ish.
When doing the same with the greater magazine upgrade, the improvement is the same duration - from 17 seconds to 16 seconds if I recall.
It seems that the fire storm ability speeds up the initial spin-up of the vengeance, making the first few shots faster, but doesn't appear to affect it's final fire-rate during sustained fire from what I can tell.
In multiplayer with Friendly Fire enabled, the Retribution skill will cause any player instigated damage (including from the chaplain himself via spear of caliban AoE) to reflect onto the shooting player by setting a bodypart to red/flashing health.
This is not normal friendly fire damage as it can happen to shielded players (such as the chaplain themselves with self sheild) and sets the resulting bodypart to red/flashing but doesn't alone break it regardless of damage done (edge of hellfire area reflects the same as a direct hit with plasma cannon).
To reproduce: Take spear of caliban as chaplain on mission with friendly fire enabled and shoot a wall at point blank while under the effect of retribution. Chest will immediately be at near-fatal levels from an otherwise minor friendly fire hit and also at a time where you should be immune to all damage.
(Experienced frequently during No Mercy / FF multiplayer gameplay, but only recently realized the cause.)