Thanks for letting me know that CEV is obsolete. I'll have to look into the details. However, I don't think our disagreement is in that area.

it's easy to say we should increase happiness, all else being equal. It's not so obvious that we should increase it at the expense of other things

Agreed, but the argument works just as well for decreasing happiness as for possible increases. Even someone who valued their own happiness 1000x more than that of others would still prefer to suffer than for 1001 people to suffer. If they also value their own life 1000x as much as other people's lives, they would be willing to die to prevent 1001+ deaths. If you added up the total number of utils of happiness, according to his or her utility function, 99.9999% of the happiness they value would be happiness in other people, assuming there are on the order of billions of people and that they bite the bullet on the repugnant conclusion. (For simplicity's sake.)

But all that's really just to argue that there are things worth dying for, in the case of many people. My central argument looks something like this:

There are things worth dying for. Loosing something valuable, like by suppressing a biased emotion, is less bad than dying. If suppressing emotional empathy boosts the impact of cognitive empathy (I'm not sure it does) enough to achieve something worth dying for, then one should do so.

But I'm not sure things are so dire. The argument gets more charitable when re-framed as boosting cognitive empathy instead. In reality, I think what's actually going on is empathy either triggers something like near-mode thinking or far-mode, and these two possibilities are what leads to "emotional empathy" and "cognitive empathy". If so, then "discarding [emotional] empathy" seems far less worrying. It's just a cognitive habit. In principle though, if sacrificing something more actually was necessary for the greater good, then that would outweigh personal loss.