I have a couple of RAF studies on fitting 12.7`s / 20mm cannon to bombers in early WW2 (lancaster etc), it states on more than one accasion that in their opinion, it was dramatically more effective to have a powered turret than a hand mounted gun of any sort. This was a discussion point because the turrets weighed an absolute ton.
Incredibly annoyingly I cant remember which damn file number it was in !!!
Wellington 1C with 1050hp Pegasus engines had a speed of 235mph. I was going to use that as a point to say that turrets came at a cost in speed. But the rather good performance of the Wellington, given the exceptional range and bombload, likely reflects the efficiency of the geodetic structure plus the effect of integrating the aircraft into the turret (rather than the other way around) and forgoing the unnecessary dorsal turret.
Nevertheless I would say that turrets did make sense until engines in the 1500hp and above range were common.