One of the problems with lightening mass to increase cycling speed is that most cocker owners nullify it by decreasing the LPR output pressure. Lightening reciprocating mass will allow you to either increase cycling speed or lower ram pressure, not both.

As for ball suckage, i had a thought last night that will easily allow us to determine how much ball suckage there really is and hopefully someone with the time and tools (AGD hint hint) can performe this experiment for me.

Attach a ball to a string. Attach the string to a calibrated strain guage or other force measuring device that does not allow the ball to move. Now rest the ball so that it is just above the bolt in a verticle feed gun with the string running straight up to the strain gauge. Cycle the gun and measure any increase in force on the ball. A decrease in force would mean blow back, although in this setup we couldn't measure the magnitude of it.

We could determine the amount of ball suckage, add gravity and we'll get max feed rate.