Federal employees are both over paid and under paid depending on the job and the quality of the employee. Doctors, nurses, lawyers, engineers, computer programmers are underpaid; receptionists, secretaries, clerks are overpaid. When benefits are factored it, the amount of overpayment becomes more marked.
However, the problem of underpayment can be a problem when it comes to attracting quality employees.
My guess is you could get a good correlation between amount of “overpaid” and jobs that traditionally are filled by liberals.