Yamataian don't need to improve the processing power of thier brains or make add ons since their telepathic abilities link them rather easily to the PANTHEON network, which is a godlike aglomeration of quantum computers spread in Yamatai (and probably several of their key worlds) with subnetworks maintained by PANTIES, KAMI-IES, MEGAMI-IES and other integrated electronic suites units using the Kessaku operating system.
Seeing the constant access to a much more powerful computer system at your fingertip, going for improvements for your own brain is sort of silly if you are a yamataian. You already have huge capacities to access and absorb information which are pretty close to the nekovalkyrja. Aside from the nekos combat OS, heightened sensory system, unit-to-vehicle interfacing system (SPINE)... the Yamataians do a lot mostly better than most Nepleslian cybernetic brain component seen so far on player characters.
Basically, if a Yamataian wants to get a significant update of his capabilities he just changes his body to the latest type. Thing is though that the Yamataian is the latest model and most geshrin played musical bodies to get the latest, most performant bodytype.