Well if you have a few hours, just take it off and find out, I'd imagine that the results are similiar between the wrx and the STi. I did it to my wrx, no need to do it to the STi just yet
The engine/exhaust is a little more pronounced, and you can hear the recirculating valve go. I believe when i did it the turbo spooled up 200-400 rpms or so quicker.
Any actual power gain was not discernable by buttdyno.