I think the faulty translations cited in my previous post show that this passage has been misconstrued. (Jowett may have it right, but he hasn't rendered it in such as way as to make it clear what the right way of taking it is.)
We naturally take Plato's discussion of pleonexia to be about competition: to have pleonexia is to get the better of someone else. It means winning out, getting one's way, and prevailing. Pleonexia, in this sense, is libido dominandi.
But Plato, rather, pretty clearly takes it to be, after all, about coincidence or disparity in judgment, which may or may not carry with it any imposing of one's will . This is clear from the general principle he gives, on the basis of an induction, at 350a:
Do you think it's the same for every branch of knowledge and ignorance? Do you think there is ever any knowledgeable person who would deliberately choose, either in action or in speech, to do more than another knowledgeable person would do? Wouldn't he do the same as someone like himself would do in the same situation? (Griffith)Notice that Plato explains the coincidence of judgment first in terms of the persons involved ('do more than another knowledgeable person') and then in terms of the actions of such persons ('do the same as someone like himself would do'). Notice, too, that he explains each of these using a counterfactual: Does a knowledgeable person do the same as another knowledgeable person would do, sc. in his circumstances? To show pleonexia with respect to someone, on this way of looking at things, is to judge that you should do something that, in some way, in its result, goes beyond what that other person would judge that you should do.
Thus, for an unjust person a)ciou=n tou= dikai/ou pleonektei=n is to judge that he should get something which, in its result, is more than a just person would judge that he (the unjust man) should get in those circumstances; and for him a)ciou=n th=j dikai/aj pra/cewj pleonektei=n is to judge that what he should get is more than what a just person would judge that he (the unjust man) should get.
Thus the correct translation of:
ti/ de\ dh\ o( a)/dikoj; a)=ra a)cioi= tou= dikai/ou pleonektei=n kai\ th=j dikai/aj pra/cewj;
would be something like:
"And what about an unjust man? Doesn't he claim that he should get more than a just person would claim for him, and isn't what he claims he should get more than what he justly should get?(or: 'more than what a just person would claim he should get')"
(I take pra/cij to be schematic: it stands for whatever sort of action is indicated by the context; in this case, getting.)