The least drag coefficient for a rough sphere is about 0.2; the ball turret had a diameter of about 4 feet (somebody fit inside), so it added about 2.5 square feet of equivalent flat plate area. The turret, its retraction mechanism, and its traversing mechanism probably added about a 1500 pounds to the aircraft directly (this excludes the structural changes needed to accommodate a big hole in the fuselage). The equivalent flat plate area of a "clean" B-17 that had a ball turret was probably about 40 square feet; the ball turret would increase that by about 6%.
WW2 bombers were running into the "flying airbrake" territory of aerodynamic efficiency, but the low accuracy of dumb bombs dropped from high altitude made mass formations necessary, so features like speed and maneuverability (see: Mosquito) were useless in a bomber, although I think a case can be made for reducing B-17 and B-24 armament, thereby reducing crew numbers and equipped empty weight, to increase bomb load and thereby reduce the number of sorties to cause the same effect on targets (removing 1,000 lb of machine guns and flight crew can increase the bomb load by 25% on longer range missions; the cumulative effect can reduce bomber crew casualties, although not reduce casualty rate)