I ran a few custom battles the other day on normal with a platoon fire guard unit and a line infantry unit. The trend seems to be the fire-by-rank unit outgunning and causing more casualties on the first 2 volleys, but the constant losses they are taking makes them start redressing their lines sooner than the guard unit, and on volley 3 casualties are roughly even. By volley 4 the line infantry is losing too much firepower from casualties and by volley 5 they're usually around 40 casualties more than the guard unit and soon routs.