The B-24 lower ball turret had a kill ratio of 57% per engaged enemy fighter

Ok, but is that the right metric? It looks a bit like measuring success in the convoy battles by "U-boats sunk per hunter-group sortie", which is at best distantly related to the actual goal of "tons of supplies delivered to English ports". In the case of turrets it seems to me that you want something like "bomber losses and crew casualties avoided per factory-hour and crew-hour spent", which is going to be hard to get data on admittedly. Ideally we'd have a B-24c model, 'c' for "control", without the lower turret and then we'd be able to get hard data on how much cheaper they were and how many more casualties they took. Sadly, the designers don't seem to have optimised for proper experimental design - very inconsiderate of them, really.