it is also about how much air could be shifted. Vs stock turbo to a GT40 at at a bar of boost, compensate half the timing to the GT40 it still makes more power. As for the same reason of why people don't install factory exhausts on a high powered cars. If the stock air box is only made for 250HP, it will obviously restrict and affect drive ability on a 500HP setup. A well made pod enclosure or a custom cold induction system will definitely help getting the best of both worlds.