Well here goes my first post ever...
I just bought an sc300 last wendesday, and it seemed a lot slower than one I test drove before. I checked the air filter and it was pretty dirty so I knocked the dirt off and vacumned it, then vacumned out the airbox which had some crap in it. I put it back in and it didnt really change the performance... Just about 3 hours ago today I stopped at a parts store and picked up the K&N drop in filter, I put it in in the parking lot and I have to say it DID make a little bit of a difference. It seemed faster, at least enough to notice. My friend in the car who was with me when I bought the car said it did feel faster too.
Other than that it is definitely a good investment due to the fact that it lasts forever, and since the cheap filter was $20- I just went ahead and spent the extra $40 for this one. I hope this helps.