I kinda like the idea. But I also agree primarily with Samin on the matter. It would be better represented as an early african religion or something, although they were certainly NOT the only tribes to practice cannibalism. Nearly all early humanity did. And they did it not for food but to consume the strength and intelligence of their enemies.
There's some basis for why they might've believed this worked aside from merely pseudo-logical imagination. If prepared/preserved properly, the pineal gland in the brain from a nearly live victim would've produced an amazingly powerful DMT phenomenon allowing shamans to enter a state of mind most humans never see until the moment before death.
And the 'meat' would've produced a common 'mad cow' phenomenon, causing a savagery to emerge in the ingester that would be akin to rabies. For as long as the cannibal could survive this way without becoming a rampaging danger to even his own people, he would indeed begin to show signs of the disease, including frothing at the mouth, reckless abandon, and a mind for little but combat.
The problem with putting all this to game effect is that, politically, it could be perceived as immoral to propose there being any benefit in this act, which by today's standards is viewed as the ultimate in sinful behavior.
Cannibalism WAS used as a food supply in some places though. For example, among the Canadian Natives, cannibalism was so rife that the people began to show drastic signs of the 'mad cow' disease one gets from ingesting a being of such similar genetics to one's own, that they called those taken by the disease by the moniker, Wendigo. (Or some would say the Wendigo had possessed these people as a punishment for committing Cannibalism, which was not entirely untrue to some extent, were the Wendigo considered the disease itself.)