What I defend is that, in the absence of a good argument based on welfare ranges and not p(sentience), we don’t know if the welfare range of simpler animals is below or above the bar above which their welfare would dominate over that of more complex animals (not that it is below!).
I suspect we disagree about how it is reasonable to apportion probabilities in the absence of empirical evidence. I think that conditioning on p(sentience) is sufficient to justify a non-negligible probability of similar levels of welfare range in the absence of any further empirical evidence. The empirical evidence that shrimp have small brains detracts from this probability, but not by much. That evidence makes it much more likely that they are not sentient at all, not much more likely that they only feel small feelings.
From your perspective now, given your uncertainty, you might have no reason to think that the drive changes things in a better or worse way, and so the expected value derived from taking the expected value of each possible consequence considered piecemeal and summed up might be neutral.
I think if you accept some degree of objective chance, this might completely resolve the problem. (I explore this idea, without direct connection to incomparability, here: https://philpapers.org/rec/SHICAT-11)
It is consistent with a purely subjective neutral expected value, however, that you might be quite sure that the results, if revealed in their full detail, would be incomparable.
This creates an intuitive tension. There is something called the “Principle of Reflection”—it is a bit controversial, but very intuitive—that says roughly that if you know you would believe something if you were put into a better epistemic position, you should believe it now. The challenge Bulldog raises relies on taking reflection seriously and thinking that we would almost surely see incomparability if we looked closely at all the details, so we should accept incomparability now.