Getting a header/dp w/o cat makes the car extremely loud, but do not get rid of resonator. Having no cat with the dp will give it a deep tone when you get on it as long as your resonator is there, but a high flow cat will have better sound quality. I had all my work custom done and it was by far the loudest cobalt i have ever heard. I had my resonator taken out but later on put a glasspack muffler in its place and it did wonders for sound quality. Since your not looking for hp, id go with a highflow cat and get whatever muffler you want. The oxygen sensor is BEFORE the cat as the cat is at the very end of the downpipe but there is an O2 senser in the catback. No cat will throw a CEL but wont effect anything, only way to fix it is tuning which is pointless to spend the money on unless your already getting it tuned. But all in all, catless header/dp will make a HUGE diffence in sound. My car idled as loud as joshs did when he got on it.