^^^ WHS.
Do not go buying new parts 'thinking' it is the
MAF or something else.
Go direct to a Subaru specialist workshop who can diagnose this ASAP. There are a lot of cheap and easy things this problem could be. There are a lot of expensive ones too!
Go see a mechanic who knows what a turbo motor is. Putting a new turbo on an old motor doesn't cause issues.
What modifications have been done to the car? Does it have a tune or is it running a boost tee for added 'phulleh siqness'?