Will have to study the sound proofing when the car is here. I've done similar write ups for BMW cars, which can be notoriously difficult & tend to dislike doing anything not easily restored to factory. I found your link helpful. Interesting how a useful grommet "disappeared" in the 2015 model year, but one poster put one where it was.
We are looking to install a Laser Interceptor system as I have on my '14 335xi. Very effective and discreet. The head sensor/transmitter units have wires which run in to the cabin's central control unit. I generally use the OBD2 port with Valentine's "Savvy" connector for the V1 as it both simplifies wiring and reduces annoying false alarms below preset speeds. We're not particularly fast, but have no great love for revenue driven speed traps. The "Waze" app is also great.
I rather liked the Forester XT. Tried a WRX, but found it just too loud and "boomy" at lower RPMs. I'd imagine the STI version is even more so.
Thank you again!