Untuned dyno's should be taken with a grain of salt. When you install an intake and reset your ECU, you're car starts learning fresh. With more air than expected, it will lean out your AFR and that equals more power. The more miles you put on it, the less the gain will be, as your ECU is constantly trying to correct your AFRs. That's why a part feels like it's added a lot of power when you first bolt it on. Then after a few days the effect is lessened.
Case in point, a local Mazdaspeed3 owner dyno'd his vehicle at a dyno day. He bolted on a Cold Air Intake and went back on the dyno. First run showed 32 whp gain and lean. Second run showed 22 whp gain and not as lean. Third run showed something like 12 whp gain.
A company who manufactures these parts can, if they chose to, publish dyno numbers of big gains. Because, you know what? It did gain that much on the first run! So technically, it's legit, right?
When choosing aftermarket intakes, always consider the design, the research behind it, the pipe diameter and the MAF housing diameter as well. Placement of the MAF sensor is also extremely critical. Long story short, I went with the K&N Typhoon, am fully tuned, and have no regrets. I feel this intake is among the best in design and build quality.