Many interesting views and good points made. Sadly little quantified data beyond the general type/campaign example (mine included!

).
I think it's fair to say that the claims against Luftwaffe fighters by US gunners are (albeit with the best of intentions) significantly greater than Luftwaffe losses, but would be interested in data either way.
The Mosquito provides a real world example for the minimum resource use as an unarmed bomber alternative to the heavies. To massively (over-)simplify, the Mosquito concept used less resources in every single way (in many cases greater than 50% less) was as accurate, and could hit a target more frequently and come back more often than a heavy - even leaving aside the wood construction. Obviously that's very simplistic, but a study of the data is interesting, and the Mozzie's existence prevents analysis dismissing the 'turretless bomber' scenario out of hand.
One point not covered so far is the right turret in the right place. The RAF bombers were mostly given four positions - nose, dorsal, ventral and tail. The ventral turrets were generally removed for drag and inefficiency, while we now know that it was the
one turret / observation point they really needed. The observation and sometimes defensive fire from the tail position was useful; the dorsal less so - and far less than retaining a ventral turret, while the nose was essentially useless - but see below.
Meanwhile the Sperry ball turret was, arguably, one of the 'best' as in fit and effective turrets ever - in the hardest position to prescribe for - most alternatives were remarkably bad.
CAPflyer's excellent point regarding British turrets being high drag is not actually as black and white as it seems. Induced drag from the Frazer Nash tail turrets vs the B-17's flexibly mounted guns would be interesting - particularly when balanced against it's worth, counting field of fire, calibre and accuracy. However the Lancaster was always fitted with an essentially redundant streamlined nose turret rarely if ever any use in night ops, and actually the exact thing the B-17A-F needed and yet they got the less streamlined and remotely fired chin turret in the G.
The dorsal turret fitted to the Lockheed Hudson and early Halifaxes was a modification of the turret fitted to the Halifax nose; while the later Boulton Paul II turret - designed for the Defiant, and used on many types, including the Halifax was not significantly draggier than the dorsal Sperry type on the B-17 /B-24 - oh, and it had four .303 rather than two .50s.
Those pointing out the post-W.W.II use of turrets might like to include the advent of radar gun laying, and the troubles the B-29s
actually had over Korea, one of the few occasions W.W.II fighter vs turreted bomber battles occurred after 1945.
Interesting discussion.