The thinking behind experimenting with a 40mm gun turret on the Wellington was to keep the day fighters out of range. Even with only 20mm cannon on the bombers the day fighters would have to stand further off until actually making an attack and could then be engaged over a longer period of time. Think of a 20 x 40mm A/A battery flying at 200 mph at low level. Ideally with other such batteries with overlapping fire.
One purpose of a formation of bombers is not only to be able to bring more guns to bear onto a single attacker but also, while the aeroplane attacked has only a head on target to fire at, the others in the formation will have a variety of angles of attack. With cannon they can engage further out and thus get, not only a longer period firing at an attacking fighter, but also a greater variety of angles of view of the attacker.
In a night bomber things could be different. On a typical NW European night gunners only normally saw a night fighter within range of a .303". Some rear gunners had a protocol not to fire but tell the pilot what maneuvre to take in the hope that they had not yet been spotted. Tracer fire would give away their position. Others preferred to use lots of tracer to scare the night fighter away. There were some who would only prefer the later twin 0.5" turrets if they had automatic gun laying 'Village Inn' radar as they could accurately engage further away ay night. Otherwise they preferred to frighten the night fighter with lashings of tracer from 4x.303" in the general direction of the night fighter even if they were unlikely to hit it.