Here's a dyno run sheet from the K&N typhoon intake for an '09 STI. You can see the horsepower and torque gains across the rpm range. The vehicle mileage and which gear the pull was done in were posted but for some reason the temperature, humidity, and pressure readings weren't put on this chart. Below the STI dyno sheet is the sheet for the MkV GTI which has the atmospheric conditions listed for that kit. I don't know why they weren't listed for the STI kit, but they were recorded. I'll bring it up so we can get the info displayed online.
You can read about the horsepower gains testing procedures here. We do three runs with the stock intake, 3 runs with the K&N intake, and the gains posted on the website are an average. AEM follows a fairly similar testing procedure, as far as I know from talking with their guys.
K&N Power and Torque Testing Information
Here's AEMs dyno pull for their 09 STI kit which has all the atmospheric conditions listed.
And links to some independent reviews of AEM intakes.
Independent Product Reviews of AEM Cold Air Intakes
Yes, intake systems aren't the most horsepower/dollar investment; however, they do help.
Hope this helps guys.