More or less what you would expect. Makes sense that a less restrictive filter also does a less good job in filtering the dirt.
However, I disagree about one claim in the article, namely that you cannot gain more power by increasing the air flow. Then why would turbo chargers or superchargers be installed in engines if they do not provide more power? More air + more fuel is more power. Also, a high revving engine with the same displacement as a low revving engine gives more power because more air/fuel flows through the engine.
I'm not an expert, but that just makes sense to me.
A supercharger or turbocharger pumps a whole lot more air/fuel mixture into the engine, at higher than atmospheric pressure. And while you do get more performance from that, in order to really benefit from it, your engine needs to be built from the ground up for it, with a more efficient manifold than stock, on carbed engines you need 2 high CFM carbs, you need a high lift long duration cam with a fast ramp profile, ported and polished heads, and open pipes. A higher static compression ratio is also necessary, requiring higher octane gas. You will get a lot more power out of all that, but it will mostly be somewhere near redline, and the engine will barely idle at 3000 rpm, even with the secondaries closed. In order to handle all that power, you will need to build your engine out of high quality parts, stock parts will break right away.
What I just described is a racing engine, purposely designed and built to run at full throttle. You do not want something like this on the street, it just wouldn't work. I consider K&N air filters to be racing parts, to be used on engines which are already purposely abused.
I have never seen any evidence that running a high flow air filter on an otherwise stock engine will increase power. If that were the case, running it without a filter would make even more power, but it doesn't (if anyone has any actual evidence that it does, I'd like to see it) It just wears your engine out a lot faster. It doesn't take Einstein to figure out that letting dirt into an engine is a bad thing. Where I live, dust storms are a fact of life. The best filter to use is the one that traps the most dirt, which is what an air filter is supposed to do. I do not consider an air filter to be a tuning part on a street engine. On a non racing street engine, I consider engine reliability and longevity to be far more important than a slight increase in power, so slight that you cannot tell the difference. If it takes a dyno to tell you that you have gained 2-3 hp, while cutting the life of your engine in half, is it really worth it? That of course is up to you. I will stick to stock parts on stock engines.
Oh yeah. The reason the damage caused by using K&N (or any other high flow air filter) isn't immediately obvious. It takes time for the damage to reach the point where you start to notice it, and by that time, most bikes and cars have been through several owners. Most bikes never make it to really high mileage. I have 70,000+ on my Vulcan 750, and plan on it making it to well over 100,000 miles. To most people, 50,000 miles is pretty high for a bike. I know of more that one person with over 100,000 miles on Honda Rebel 250s.