thats not proof for me i would rather see dyno runs from your own car with standard airbox with a clean filter and then the hks kit on it.
15% is a huge gain from something that. and please dont say it was in max power as i remember reading induction kits giving 10% on a 1.2 corsa. yup gave the power but the difference was old filter against new one, bonnet open with the induction kit, and ice on the manifold
done some quick googling and found the civic kit is supposed to give 8-10bhp - real life dyno showed 3 at the wheels and a bigger flat spot.
and on a 370z average gain is 10-12 at the wheels with the biggest being 17
so sorry cant see how a engine with half the capacity and designed more towards economy could get that