You may expect, that without any brain tissue to study directly, there is no way to rate the complexity or uniqueness of the sperm whales brain within the cetaceans, but there is! Genes can tell quite a story by themselves. Even though we don’t yet have the technology to guess how each mutation might affect phenotype, we can measure the selective pressures upon them, then compare it to other related species, and against pressures on other genes within the same species. That can prove just how special a brain is becoming even in the absence of other information. Here is how this was done in humans.
If any reader here has access to the relevant database and analytic programme, then within days we can have an answer. Now I know that it is quite an investment in risked time, but I must remind you that the reward could the greatest scientific discovery of all time. My advise is just do it.