That turret system was actually pretty lousy, the periscope sighting was a joke, so
heavier guns wouldn't have made any real difference.
Imagine if you would sitting in a pitching and rolling aircraft while staring through
the limited field of view of a periscope at moving things that are
below you,
while your seat rotates.
Get the picture?
The Sperry belly turret on early model B-17Es had the same limitations, that's why
the US went to the ball turret. A Bendix retractable remote control turret was used
on the B-25B through G, but it was most often removed in the field because it was
next to useless.