After an "internet tune" with mixed results, I decided to tune the car myself (Previous knowledge of tuning a JDM STI V3/V4 with Power FC). What I've come to realize is that what people are getting is not a "tune", but rather a map that restores the performance to the levels intended by Subaru. Driving on 95 RON, my car would typically end up with an IAM <0.5, which makes the engine perform like a 90 bhp Civic.
IAM was being dropped by the ECU due to rapid AVCS and ignition timing transitions, and in general a way too aggressive ignition map, especially at peak engine torque, which caused detonation. As the mid-range detonation was solved, the engine would start to detonate at higher rpms - as it had never been using IAM 1.0 for an extended period of time.
Next up was AFR. Stock map programmed to run AFR 14.7 at all to high loads and rpms, and switch-over from CL to OL was way too delayed - causing lean detonation BEFORE reaching OL. Lowering the AFR above 1.0 load and 1900 rpm resulted in a very noticeable increase in torque. Better acceleration and engine will hold on to 5th gear on steeper inclines.
Equalize cruise and non-cruise AVCS and ignition tables, to avoid rapid changes on cruise-non-cruise transition
Reduce timing overall, but especially at peak torque
Zero out CL to OL delay
Decrease AFR at load
In practical terms the ECU is de-tuned, but this restores the intended performance which is hidden away by IAM < 1.0.
Secondly, mixed fuel consumption decreased by as much as 20%!
Wonder what genius at Subaru that made the stock maps....