So far my setup has been just about perfect. It feels like my engine likes the IS20 more than it did the IS12 it came with .
It's not a bad idea to go with a downpipe, and you will definitely be able to make more power, but so far it hasn't been necessary for me. Another reason I haven't put a downpipe on yet is because my car is registered in California which has very strict emissions laws. It might be a good idea to do a downpipe swap at the same time as the IS20 if you aren't concerned about emissions because it shouldn't cost any more in labor.
It's not a bad idea to go with a downpipe, and you will definitely be able to make more power, but so far it hasn't been necessary for me. Another reason I haven't put a downpipe on yet is because my car is registered in California which has very strict emissions laws. It might be a good idea to do a downpipe swap at the same time as the IS20 if you aren't concerned about emissions because it shouldn't cost any more in labor.