I'm running stage 2+ hardware and tune with the factory air box until my issues with the Racingline intake are resolved. Works fine.
Shoduchi's findings are inline with what Racingline quote for their intake and if you don't like loud intake noises, it's the kit to get - pending some ongoing investigations into it's fitment issues! You won't see those kind of gains until you are past 4500ish rpm. Below that there is no change, so it depends how you drive the car. K03 is all about midrange, so 5000+ action won't yield masses more power at stage 2 levels.