I tried an experiment. I set up a custom battle with 3 cossack musketeers +1 spearman, vs 3 crossbowmen.

1 musket unit was in default formation.
1 musket unit was 5 ranks deep.
1 musket unit was in default formation but blocked by a spearman.

The unit blocked by a spearman did not discharge arms directly into the backs of the spearmen. In close zoom you could see the guys on the edges with LOS fire, but the guys in the middle who were blocked held their fire. About 20 spearmen died, and almost all of them were near the edge. This unit of musketeers spent much less ammo than the others.

The unit 5 ranks deep gave a great performance. It had a higher rate of fire than the default formation. I'd say the time between volleys was about 30% less. The total amount of ammo expended was less. And the kills were about the same as for the default unit.