Although our Airbox systems are super smooth, you should STILL feel (or better still MEASURE), a 25 hp increase, it's just that it will rise to that more gradually (relatively), than it would with a DP kit. By more gradually I also mean more manageably and I'm only talking about fractions of a second for the full power to be achieved.
If you are not happy with the improvement as it is now, there are 2 possible explanations;
1) Something is stopping the system delivering the full potential of the jets. This means things like inadequate bottle pressure (which could be due to low temp or low bottle content volume), incorrect nitrous to fuel mixture ratio (excessively rich), inadequate spark strength (very common), slipping clutch and don't think that just because it's not obvious (instantly hitting the red line but not going anywhere), that it's not happening, as 99% of production motorcycles make more power than the stock clutch can handle and slip BY SOME PERCENTAGE, even without nitrous. If you read the appropriate section of my book, you'll find all this info and much more, along with the ways of overcoming them.