Fixed.

I cleaned the MAF - no change.

I pulled the lead on coil #3 - no change (which made me suspect bad coil #3)

I pulled the lead on coil #1 - worse idle (which reinforced my preliminary diagnosis)

To confirm, I swapped coils #1 and #3. Problem was that the symptoms did not follow the suspected bad coil #3.

I then checked the passenger side. Immediately I saw the fault - the lead on coil #4 had partially unplugged. Replugged the lead and SCORE. My STi is now purring and boosting as it should.

Interestingly, even after pulling leads from the coil, I ran the obd scanner (with the lead still disconnected). No CEL and no fault codes.