I interpret the brain size data differently. That a cow is not smarter than a mouse shows that most of brain size is used up controlling a larger body. That a whale has a large brain shows that large brains work. Animals with slightly larger brains than predicted by body size are smarter (apes, dolphins, ravens). This can probably be pushed further.
(I don't understand why a large body should require extra neurons. One explanation is that resolution of touch is independent of size. Is this true? Why would a large animal need fine resolution? This would only explain a 2/3 exponent, not the observed 3/4. Neurons for controlling muscle probably scale at a full rate, but they needn't clog up the brain.)
It looks like having more brainpower controlling muscles is all sorts of useful, for example determining how finely you can do fine motor control.
Article in current Scientific American (first para and bullet points, rest is paywalled).
Podcast by the author (free).
The author, Douglas Fox, argues that there may be physical limits to how intelligent a brain made of neurons can become, limits that may not be very distant from where we are now.
He makes evolutionary arguments at a couple of points, suggesting that he is talking about how smart an organism could have evolved, rather than how smart we might make ourselves; he certainly isn't talking about how smart a machine we might create out of different materials.
From the podcast (I don't have access to the article):
Four routes to higher intelligence, which he argues won't get us very far:
He's described simply as an "award-winning author", but I don't know if he has any scientific background, and there are too many people of the same name to Google him.