Do you really think they would have installed cockpit armor and added all that weight to the aircraft unnecessarily if it was not saving lives? Replacing a shot-down aircraft is easier than replacing an experienced pilot.
The additional armor certainly provided protection against shrapnel from flak and perhaps .30 caliber bullets but .50 - 20mm AP at combat distances was going to punch through it.
I recall in one of his books, Robert L. Scot mentions either himself or maybe another pilot getting rivets taken out of his back.
They were only superficial wounds caused by the rivets which apparently attached the seat to the armor were knocked loose by bullets hitting the back of the seat.