It gets better. When deflection firing bomber guns you don't lead the con, you trail it on a shot behind the 3-9 line. The velocity imparted on the bullet from the motion of the bomber causes it to hit a point further "forward" than you think. I don't have the reference handy at work, but the sights on bomber guns were calibrated so you know how much to trail the enemy fighter based on deflection angle, with a 90 degree deflection requiring 3 rads and a 0 degree deflection requiring 0 rads.
As for speed, using ground speed as a reference, yes, the bullet going out the tail gun has a lower velocity than a bullet from the nose, but then you have to figure the speed of the aircraft that's racing to meet it. Bomber speed of 400 fps, fighter converging at 500 fps.
Fire a bullet at 1000 fps. Relative to a fixed point the bullet is traveling at 600 fps. Relative to the bomber is is traveling at 1000 fps, relative to the poor schmuck behind the bomber the bullet is traveling at 1100 fps.
If that same fighter is in front of the bomber, with the same velocity, and you fire from the nose guns. Relative to the fixed point the bullet is moving at 1400 fps, relative to the bomber it is moving at 1000 fps, and relative to the fighter it's moving at 900 fps. Given the lower relative speed from the front it will take longer to traverse the same distance, increasing drop, drag, and stability decay.
HTH. HAND.