i'm assuming since you mentioned the 2.5L you are referring to the USDM STi intake manifold.
Let's get some stuff cleared up first:
The usdm STi is drive by wire, and if you choose to use that intake (it houses the throttle cables) you will have to do some modification to get it to work on your WRX. (what exactly i'm not sure, as i've never completed one)
Next, USDM STi's STILL HAVE the tumbelers in the housing. And they sit below the intake, they're not all one part like the JDM V8's intakes. So if you bought just the intake, and not the housing that sits below it, you'll essentially have a red painted wrx intake that doesn't work on your wrk because it's drive by wire. phew.
BTW the V8 JDM intakes are the ONLY ones that do NOT have the TGV"s in them from OEM.
Next, i know with my street tuner i can simply disable certain CEL's. I looked up all the codes i am allowed to disable, and guess what, TGV is one of them
may want to look into that, you can get your EM and not drop duckets on trying to defeat some code with expensive fixes all in one shot. There's simply a screen you go into and check certain DTC's, then the code is gone next time you flash. The power of working with the stock ECU