This strikes me as a pretty poor argument. The problem, of course, is that utilitarianism does not assume that any such super-person exists. Rather, the theory rests on other grounds - namely, the notion that each person's interests matter equally. As Hare writes:
It is indeed rather mysterious that critics of utilitarianism, some of whom lay great weight on the 'right to equal concern and respect' which all people have, should object when utilitarians show this equal concern by giving equal weight to the equal interests of everybody, a precept which leads straight to Bentham's formula and to utilitarianism itself.
It should hardly be necessary to spell this out. To have concern for someone is to seek his good, or to seek to promote his interests; and to have equal concern for all people is to seek equally their good, or to give equal weight to their interests, which is exactly what utilitarianism requires. To do this is to treat others' interests in the same way as a prudent person treats his own interests, present and future... To do this is not to fail to 'insist on the separateness of persons'. (R.M. Hare, 'Rights, Utility, and Universalization: Reply to J.L. Mackie', in R. Frey (ed.) Utility and Rights, p.107)
Nozick's objection echoes Rawls:
To use a person [for another's benefit] does not sufficiently respect and take account of the fact that he is a separate person, that his is the only life he has. He does not get some overbalancing good from his sacrifice (Nozick, ASU, p.33.)
But this is foolishness. No-one is claiming that the demanded sacrifice is for his own good. Rather, it is to benefit someone else in greater need, another person for whom their life is the only life they have. Nozick's egoistic objection is patently question-begging. Further, as G.A. Cohen asks, "if such sacrifice and violation are so horrendous, why should we not be concerned to minimize their occurrence?" (S-O,F&E, p.32) (This echoes Parfit's argument that common-sense morality is self-defeating.)
There is, however, a more charitable interpretation of the objection. We begin by noting that utilitarianism makes the normative claim that boundaries between lives lack moral significance (rather than the absurd descriptive claim that no such boundaries exist!), and we ask why this is so. Rawls' objection, then, is that the utilitarian's methodology (appeal to the impartial spectator who sympathetically identifies with everyone) leads them to neglect the descriptive fact. If the ideal spectator imagines himself as everyone at once, he might forget that they are different people, and that benefits to one will not compensate another for the harms they suffer. If this were the explanation, it would undermine the utilitarian's normative position. But, as we have seen, this is not the explanation -- as is obvious from the fact that many utilitarians employ the methodology of a detached rather than identifying spectator, and there is no reason why the former should suffer such confusion.
So, cutting to the core of the issue: Can it be right to burden someone merely to benefit someone else? Derek Parfit clarifies the issue:
We can first distinguish two kinds of weighing. The claim that a certain burden factually outweighs another is the claim that it is greater. The claim that it morally outweighs another is the claim that we ought to relieve it even at the cost of failing to relieve the other. Similar remarks apply to the weighing of different burdens, and to weighings of burdens against benefits.
An obvious case of a benefit factually outweighing a burden would be if everyone would prefer to have both rather than neither, i.e. if they were willing to undergo the burden for the sake of the benefit.
Now, it's just plain silly to deny that we can make interpersonal comparisons here. If I get a papercut and you get your head chopped off, it is absurd to deny that you have suffered a (factually) greater harm. And it is similarly absurd to deny the moral counterpart, that it is more important to save your head than my finger. Egoistic complaints about "compensation" are irrelevant here -- of course saving your head will not compensate me for my papercut. But it can still be of greater moral weight. As Parfit rightly points out, these are two very different concepts, which proponents of the "separateness" objection mistakenly conflate.
As a final point, I think the issue of incomplete relativity also rears its head here. The "separateness" objection implicitly concedes that benefits can outweigh burdens for a single person. But why should me-now be forced to suffer for the sake of my future self? If I'm a bizarre 'aprudentialist', who cares only for my present well-being, then such impositions seem no more legitimate merely because I am also the (eventual) beneficiary. The separateness of persons thus seems irrelevant to the issue of utilitarian sacrifice. In the end, I think the objection really stems from an illegitimate attachment to the thesis of self-ownership. (And we all know where that leads!)
Update: I address a more sophisticated version of the objection here.