I look at what the factory was targeting, and the intent when creating a particular "engine package". I really don't believe HP is the deciding factor. Although the 273 "commando" wasn't at the mountain top in HP, it was designed to compete in the Pony car war and the autocross races. That makes it a "performance engine".
Also, many of the "performance engines" will be surrounded by support to the car. Such as the rear end, larger brakes, sway bars, or other such features.