There's more to do with length of pipe, bends, diameter ect... they all change the way the airspeed and volume enter the turbo. A classic example is dyno a car with stock intake then with no intake... power usually always goes up with no intake, but obviously lag becomes a factor due to sucking in hot engin bay air, but its a good basic example. I always rate the under battery intake to a max of 350/360wkws... Ive seen a few go higher, but only a few and theres hundreds of these intakes out there. I sell ALOT of 4inch intakes to guys that have mine, or plaz or PW 's underbat intake and always hear back positive increases and seat of pants improvements. If you dont make power then there's usually another issue that's holding power back. Basically the more air volume you can get into a turbo, the better it will compress the air and force more into the motor, which is why the bigger intakes work so well. It can looked at like exhaust systems, the turbine outlet is only 3inch, yet we need more than a 3inch exhaust system to make decent power. Turbos work best when being fed alot of air, and not restricted back on the outlet... like Ive been saying for along time on here, more air, more flow, more power.