Not much really, as the platform was already there. I swapped the turbo (after mine died) to a VF39 (which is stock on the STI), added an STI intercooler to keep the turbo cool, and a Borla hush exhaust (as my tune calls for a 3" cat back exhaust). I changed the tune to adjust for the larger turbo, but that is not much. It's a plug and play from Cobb.LOL... he got you on that one.
So what was the upgrade that gave you the most "boost"?
When I bought it the prior owner already had an STI exhaust manifold, Helix uppipe, helix high flow downpipe, and a Cobb access port mated to the car.
For handling, which I care more about, I lowered it onto WRX struts to bring the CoG down. Installed STI OEM sway bars F/R, and STI OEM steering rack to increase the ratio and have a steady ratio all the way through, and poly bushings on the rack and shifter. I also swapped the OEM wheels for the 17x7 FXT Sport OEM STI wheels as they are lighter and wider, and wrapped them with Continental Extreme Contact DW tires. I intend to get snow tires if we drive this car up to Tahoe.
Beyond adding some sound deadening because Subies are loud and have minimal sound deadening from the factory, I have not really done much else.