No intake on the planet is going to legitimately net you 15-20 whp while keeping fuel trims happy (no need to post the link, I've read about the stillen one (we're a dealer of theirs as well
) Yes, you may see a peak gain of that much power at some point in the rpm range, but it will be at the expense of fuel trims. In the short term it means the car will see some nice gains. However as its driven and the ecu dials things back, those gains will diminish. Ultimately some form of ecu work will be needed to keep the ecu happy and the gains more consistant. Certainly a nicely made intake that will compliment other basic bolt on mods