After Installing aftermarket intake systems on all my cars I have owned from 2008 350z HR, to 370z, and now my 2011 G37. From Injen to Stillen to K&N!
Nothing is going to give you that cleaning power of OEM filters with no lost of lowend!
I have been thinking hard about the OEM intake system and the design is perfect! it sucks cold air being rammed into it from when the car is moving and it never gets heat socked at 100F. The OEM filters are very well made and hold up to all dirt! I think most people who have switch over from OEM to aftermarket and they feel all this power and better throttle is because the filters are new! if you would to replace your OEM filters with new ones you feel the same way! Also because you are now having to press harder on the Gas to get the car going you think it feels better and pulls had!
Question??
why do you want your car to pull hard? that would mean you are having to floor the car more and eat more gas just to make more HP with a open filter then OEM?? right..
Because that's why you buy a sports car, for the acceleration... 
So I'm like why do I want my car to pull hard!! I don't! I want to use my foot and eas on the gas not harder to eat more gas to feel more power to think I am getting more power!
I know this has been said but OEM is the best! and also keeps your throttle body from getting dirty with oil and dirt from the aftermarket filters!
If you don't over-oil your filters, this won't be a problem, so it's operator error. Not a fault of the filters/oil.
So if any HP is to be had with our cars it not from intakes it's from doing a highflow CBE! I bet my OEM intakes with new filters with CBE would make almost 300WHP with a tune!
For all the people who feel more power with Stillen G3 you are just leaning out your A/F and making your Eng run rich when you are above 6K RPM.
Hence a tune to level it back out again.
The HP numbers like 15/20 HP is when you are on a dyno you are running the car at almost Max eng speeds! and because the filter is open yes it will suck more air and your Eng will burn more air and gas!
If you could dyno the stock intake system with new filters going down the road at 40Mph because it's ram air design you would see more HP then the car is just running on a dyno!
So what about the fact that when running a dyno, they have fans blowing air at speed, comparable to being on the street? Forced air just like on the street.
So yes I'm going back to Stock! and never again will I invest money into a aftermarket intake! My money would go for high end CBE and tune!