K&N air filter
For several years I ran a K&N air filter in the stock airbox. Performance-wise, the K&N filter did not really show any significant increase in power, and a marginal increase in noise levels with the resonator still in place. It felt like there may have been a bit more power in the mid to high rpm ranges, and possibly a bit of a loss at the low end, but only a dyno run back to back vs. stock would really be able to tell. The difference was small, either way.
However, I am of the opinion that there is really no truly accurate way of measuring an engine's output, or more importantly, a gain in output from a modification, unless the tests are done in a controlled laboratory-type environment. There are far too many variables (engine temps, ambient temps, humidity, altitude, state of tune and/or wear and tear of an engine, dyno calibration) for your typical dyno to give truly accurate results.
That said, the only remotely accurate way for regular folk like you and I to measure gains in output is on a dyno (a precision acelerometer, such as a G-Tech, is even less accurate, but also useable). So we must live with the inaccuracies that come with the typical dyno or G-Tech and base our opinions on that.
So what am I trying to say? Only that small gains (say 4 or 6 horsepower) can come from an air filter, or simply from a cool, crisp day, and will be difficult to notice. Larger gains (10 to 20 horsepower or more) will definitly be noticable, and will only come from smart mods and tuning.
Case in point, I know of at least one Si/SiR owner who dynoed their car before and after installing a K&N drop-in like yours and mine, and saw horsepower losses. Does this mean the K&N makes your engine lose power? I don't really think so. I think the difference in power is so little that a simple variable (such as the difference in engine temperature from one dyno run to the next) could mask any gains the K&N had, and even make it look like a loss in power.
What you do get when you purchase a K&N drop-in is a high quality, re-useable, freer-flowing air-filter element that retains the debris and water protection of the stock filter and airbox, with a slightly meaner induction growl and possibly small gains in horsepower. All for a relatively low price.
For those who want a little more power, the AEM cold air intake (CAI) has been known to make power on Si/SiR motors. This link shows a '98 GS-R that saw an 18 horsepower gain from just an AEM CAI. Now, his case seems to be pretty exceptional, but 10 to 15 horsepower should not be out of the question on a B series engine. I am currently running an AEM V2 CAI on the SiR, and in conjunction with an enlarged header collector, Carsound cat and stainless exhaust system, saw in increase of 16 horsepower, as recorded by my G-Tech.
This is my second K&N drop in fiter. The first was for my old '94 Si coupe. Unfortunately, the airbox for the '99 SiR is different than the '94 Si, even though the '99 SiR shares the same airbox as the '99 Si (U.S. EX). If you are in the Toronto area and have a '92-'95 Civic Si, I can sell you a slighly used K&N drop-in for cheap...