I can only say what I saw in my theocratic career. I know well or knew, one has died, two of the members of the WBTS, long time missionaries and one a branch coördinator. I am 100% sure they did NOT benefit from that position. Poor as rats like the rest of us in our assignment.
Then, when we worked in B and in P we got to know in our congregation some of the GB well. Not that we were that kind of special but some of them really appreciated long time missionaries. Same thing, not any signs of getting money out of their responsability. There is a small group though that was in our congregation, higher ups, that I have never ever seen in the hall. I wondered why at that time.
That is all I can say. If a brother out of respect gives a gift to a particular full time servant, even a nice watch or a car (a Canadian gave us a car for instance, an old Ford though) I have no problem with that. But that doesnt mean they benefit from the organisation.