Ha ha - that is a good one. From my own experiences and that of friends (some with over 30+ years in state and federal government) the break down is more like this:
50% are absolute deadwood and worthless. They could be fired/laid off and no one would know the difference.
25% are marginal and if you hold thier feet to the fire and/or sit on them you might get some work out of them
25% are actually good workers and pull the slack for the other 75%
Your estimate is way too high. More like 20%, I think. Private business really isn't much better. Companies with more that 100 employees probably have on the order of 10% deadwood. The larger corporations rival government for deadwood ratios. The same proportion of good people doing all the work applies to business also. I see it every day. Read a few dozen Dilbert cartoons. Scott Adams knows.
Of course, certain cities have a much higher deadwood ratio. Cities where government employment is used as a form of welfare, such as Washington, DC and Batimore. In those cities, your deadwood ratio may be too low. Lets reform, then, and allow supervisors to get rid of the deadwood. Cutting salaries across the board only encourages the best people to leave government to the less than competent.