I agree with you that we need a broader basis of comparison. In Quebec, the legal profession also includes notaries. I know a great many notaries, particularly some young ones, who don't have a very high income, compared to what lawyers earn. If you take that into account, as you say, it tells you much more about what people in the legal profession are paid.
Other than deputy ministers, would it not be possible to include all public servants who practice law? I know people in Quebec who have become Superior Court or Appeal Court judges after simply working as a public servant at the Department of Justice. Why not also consider law professors?
So, there is definitely a need to broaden the basis for comparison of all people involved in legal work.