I agree with everything. But two points. First you cannot measure productivity per employee, it is incredible complicated. Second the difference is not going to be as big as you would expect to have huge differences in terms of salary (like 20% or more). So basically I think when two people with the same job positions have a very different salary you have a clear bias to give one more than the other. If they are really different in their productivity you might consider to fire the less productive or give the other person a better category (in the companies they have Junior I Junior II, Senior I, Senior II, etc).