I haven't had it on a dyno or anything to measure the performance. But I think there is a noticeable difference, but mostly at higher revs. The throttle seems a tad more responsive, and it feels like the engine revs easier at higher RPMs.
When driving the roar of the induction doesn't really kick in till towards 3000rpm by the way, which I quite like. It's also not a problem on motorways and at higher speed. You get a bit of a roar when you put you foot down to overtake something, but there's not a constant drone. Hope that helps :)