I already addressed that uncertainty in my comment:

Yes, it is plausible that once your nervous system becomes simple enough, you no longer experience anything that we would classify as suffering, but then you said "human suffering is more important", not "there are some classes of animals that suffer less".

To elaborate: it's perfectly reasonable to discount the suffering of e.g. fish by some factor because one thinks that fish probably suffer less. But as I read it, someone who says "human suffering is more important" isn't saying that: they're saying that they wouldn't care about animal suffering even if it was certain that animals suffered just as much as humans, or even if it was certain that animals suffered more than humans. It's saying that no matter the intensity or nature of the suffering, only suffering that comes from humans counts.