Although the intake may bolt up to a non-USDM market car, all non-USDM FiSTs use a completely different tuning strategy. Non-USDMs use a MAF sensor unlike the USDM cars which use a Speed Density strategy. With a MAF based strategy, any time you change the way the air comes into the engine, it greatly affects how the engine runs. This is because the MAF sensor expects a certain airflow. When that airflow is changed by a different filter element, changes in tubing bends, or by different diameter piping, the MAF sensor must be properly re-calibrated to "see" those changes. Running an intake for which the mapping is not properly designed would explain the rough idle issues. I would not recommend driving the car without having it tuned. I'm not sure who all on the market offers a tuning solution. Maybe Mountune? Hope this helps.