Hey lex I don't know if you read these but I read a book 20 years ago called Big Brain that I think you will like because it addresses many of the issues you as have talked about here, it's not a book just on intelligence it's actually an anthropological study about the boscops, who had very large brains compared to body size, and with the correlation being as it is we can only assume this translated into higher mental ability, yet they went extinct.