There are definitely some examples of lost and damaged fighters as a result of rear facing guns. For example, famous Japanese ace Sakai Saburo was seriously wounded by rear gunners of a flight of SBDs. However, it's very difficult to compile a statistical measure of the effectiveness of rear turrets.

One difficulty is that bombers usually flew in formations and an attack by fighters from the rear was generally met by fire rear facing guns in several planes. Tallying the accounts of many gunners into valid statistical counts of planes damaged or destroyed was very difficult. For this reason, there are no highly publicized accounts of "tail gunner aces", it's too difficult to sort out who hit what when a formation of bombers defends itself.

Probably the most important quality of rear turrets is that they make a "zero deflection" shot from directly astern vastly more difficult and dangerous. Many fighter pilots instead chose a safer "high deflection" shot from another angle, which is a much more difficult feat.