Thanks for the interesting post ! 

I'm very sympathetic with what you call hedonism via objective value. I agree that hedonism does not fit very well with our normative intuitions. And I also agree that the notions of objective value or irreducible normativity seem dubious and meaningless, but I think hedonism is our best shot at making sense of them and resisting normative anti-realism/nihilism: if there is objective value at all, then pleasure and displeasure, understood as phenomenal states, are by far the best candidates to have it, and nothing else comes close. (And from there we can use the anti-nihilist wager to let hedonism guide our ethical behavior.) I'm not sure you really address the idea of hedonism as a "last-chance" normative theory, which is precisely what I find most compelling about it. 

I agree that this version of hedonism is conditioned on realism about phenomenal properties, and even non-reductionism (phenomenal properties are not reducible to physical or functional properties), because if pleasure and displeasure were just some sort of physical or functional properties, it would be hard to maintain that they possess the weird characteristics that objective value is supposed to have. 

The problems of how objective value can influence us and how we can get reliable access to it are extremely puzzling, but similar problems are already faced by dualism and other kinds of non-reductionism about phenomenal properties (phenomenal states are causally inert, etc.). So it would a small extra step to embrace hedonism, at least for people who find non-reductionism appealing.  

I’m glad to see other people following this line of argument :)
I agree that many true payoff functions are likely to have high-hanging fruits, e.g. any cause area aiming at social change that requires reaching a breaking point would have such a payoff function. 
However, it's expected payoff functions we're interested in, i.e. how we imagine the true payoff function to look like given our current knowledge. I’ve also thought a bit about whether there would "high-hanging fruits" in this sense and haven’t been able to come up with clear examples. So, like Harrison, I would take issue with your second claim that "high-hanging fruits may be prevalent". I cannot think of any cause area/interventions which could plausibly be modelled, given our knowledge, as having high-hanging fruits on a sufficiently large scale (unlike the examples in your footnote 3, which have increasing marginal returns at a very small scale). 
This is because when we take into account our uncertainty, the expected payoff function we end up with is usually devoid of high-hanging fruits even if we think that the true payoff function does have high-hanging fruits. This happens, for example, when we don’t know where the threshold for successful change (or the stairstep in the payoff function) lies. And the less information we have, the less increasing marginal returns our expected payoff function will have.  I think this applies very much to the two possible scenarios you give.
The most promising examples of expected payoff functions with high-hanging fruits I can think of are cause areas  where the threshold for change is known in advance. For example, in elections, we know the required number of votes that will lead to successful change (e.g. passing some law). If we know enough about how the resources put into the cause area convert into votes, our expected payoff function might indeed have high-hanging fruits. (However, in general we might think it would be increasingly harder to "buy" votes, which might imply diminishing marginal returns.)
In any case, I would also be very interested in any convincing real-life example of expected payoff functions with high-hanging fruits. 

Thanks for this very interesting post !

I've been thinking a bit about examples of causes and interventions with increasing returns (I'm actually working on a philosophy paper that touches on this issue), and it seems to me that many examples could be found in causes and interventions that involve social norms and politics.

For example, suppose you are putting resources in a campaign to encourage members of Parliament to vote in favor a certain law, which would have a great impact if passed. There may be increasing returns of campaigning at the point where the campaign succeeds in convincing the majority of members of Parliament to vote for the law. This is because there is a threshold: if you do not spend enough resources in the campaign to convince half of the members of Parliament, the law is not passed and your impact is very low; but as soon as you reach the threshold of resources necessary to convince the majority, the law gets passed and you have a very high impact.

The same happens with social norms. Some social norms correspond to equilibria which are hard to modify, so a critical mass of efforts could be necessary to shake them, and then it becomes easy to shift them towards other equilibria. For example, if you want to spread the moral norm of antispeciesism, there might be a critical mass of antispeciesists necessary to make antispeciesism mainstream, and speciesism blameworthy in society. After the critical mass is reached it might become much easier to make progress.