My old Bh tt didn’t have a vacuum leak,but because the vacuum reservoir hoses were pinched the reservoir couldn’t reach full vacuum,causing intermittent sync problems between the twin turbos.No check engine lights.Was suggested by a good Subaru mechanic to check vacuum hoses,especially the plastic pipe/hose that runs from the reservoir around the front of the radiator to the solenoid box.Maybe a small vacuum leak causing the reservoir to loose vacuum overnight.As soon as car started,vacuum replaced.These cars are getting old and require a lot of maintenance because of the complexity of the tt engine.Cheap to buy,expensive or time consuming to maintain.
My old Bh tt will make full boost from cold.I’m always telling my son to warm the car up first.