'96 Legacy EJ22 auto. When at partial throttle, it accelerates just fine. Anything past 3/4 throttle and it bogs and won't accelerate. You can hear the growl of the engine through the intake and still sounds smooth, no bucking or backfiring. Just changed the fuel filter. The check engine light is for some emissions thing and has been there since a shop did an engine swap but it ran great for several thousand miles after the swap. Where should I start and what are the most common reasons for hesitation? A search brings up a whole lot of turbo tuning problems... but nothing referring to n/a engines.

Vacuum leaks are a typical cause.

Pull the codes anyway, could have something new. Post up the code numbers.