I worked for race teams for years. One Toyota Atlantic teem I worked on the driver thought the car was lossing power from the start of a race to the end. So we did leak down checks when we got new engines in. 3% leak down- New. At the end of a race we checked it again and we got 29% leak down. So we sent the head to get the valves done and checked the leak down when the head returned. 4%- Leak down. At the end of the next race 31%- leak down. So we put three dabs of grease down streem of the air filter. "K&N" The next engine came in with 3%-leak down new. And at the end of the race 32%- leak down and the dabs of greas were full of grit. So we changed the filter to a oiled fome filter. 3%- leak down new and at the end of the next race? 18%-leak down. Better still not good. So we made a new air box and used a Mustang GT stock air filter. 3%-leak down new at the end of the next race? 1%- leak down yes I said "1%". The rings bed in and the engine seals up. We went from changing a engine at 300 miles with 29 to 34% leak down to. Changing the engine at 800 miles with 7% leak down.
So the bottom line is. You mite gain a littel more power (1 to 2hp) but it will not last long and befor you know it you will have a LOT less.
I work on 944s only and when I tare down a 944 engine that has had a K&N filter the cyclenders have a LOT of scores from dirt that got through the filter.
If you dont beleve me do the grease test your self. Take out the filter and put a dab of grease in the air box. Then put your K&N filter in and dive the car for a few weeks and pull the filter out and check the dab of grease.