mate I've had the exact same symptoms as you - stumbling idle and missing beats when under way on full load/max torque rpms. Done plugs, intake smoke test, cleaned MAF, IAT, reseated all plugs eg crank angle/ cam position and rotated a new coil pack through each cylinder for no change. Had the car logged by European autotech and it turned up nothing. But when I ran a can of Subaru 'top engine clean' through it the other day which shows up exhaust leaks (whilst giving the head a steam clean), smoke was pissing out of the exhaust manifold to turbo gasket. I have the K03 though - don't know if the K04 is as prone to leaks.
Its $17 bucks sprayed straight into the inlet manifold where the SAI/N249 connects - cheap and easy and might turn up a manifold/turbo/exhaust leak that is changing the o2 content of the gases prior to the o2 sensor causing out of range readings for it or something?
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