Physics seems like a weirdly low-level thing if we're thinking about whether or not animals' behavior could remain the same and their subjective experience could be a state of what we would call suffering in some possible world. I just don't think that you could make edits on that level and have changes that are so fine-tuned. Editing physics doesn't leave everything alone except subjective experience; editing physics breaks fire.

And if we have to settle for a physics that leaves at least most everything the same and life remains possible, then why wouldn't we expect reward mechanisms to evolve and for the default state of affairs to be one in which adaptive experiences 'feel good' and others 'feel bad'?

gjm called this a false dichotomy, but I think a better though perhaps more complex way of putting it is that you're mixing up your multi-level maps. Take free will and determinism as an example. Some people become fatalists because they think that determinism contradicts their idea of what possibility means. But they're contaminating their maps. You have a low-level map of physics where everything is lawful and there is no thing similar to what you might call 'possibility.' Then you have a high-level map of decision-making, and the fatalists take the lawfulness from their low-level physics map and draw it onto their high-level decision-making map, and say, "Well, the lawfulness overrules the possibility", and then they start making null decisions, but that's wrong. Possibility is a primitive notion in your high-level decision-making map and only in that map, just like the laws of physics are primitive notions in your low-level physics map and only in that map. In the territory, your brain runs on physics and your decision-making algorithm runs on physics, and your decision-making algorithm computing the output of logical nodes making decisions other than the one that it must make makes you feel possibility, and the whole process is lawful, and you have a bridge map between these two levels of maps; but the map is not the territory, and that goes both ways, and your high-level decision-making map is not the territory either, and sticking physics into it is like mixing apples and oranges. And even if that seems counterintuitive, I would say that any other policy is wrong, because the fatalists lose and the compatabilists win.