Peak for peak, it's only around 5hp, but it makes much more under the peak than the GT. If you drive a stock GT and then a stock Bullitt it feels like 40hp difference, but only because of where it makes power. The Bullitt is just a better overall car. I've owned both. It accelerates better, looks better, handles better, stops better and is more comfortable than a GT.
The intake manifold really shines once you start adding power. You get more gains from everything that the typical GT owner without having to upgrade the intake/plenum/throttle body. The intake is overkill, but that really helps you as you start modding.