I would select a couple of tuners, Freektune, Panda, FSWerks, Stratified etc. email them your situation and see what they recommend and go from there. Generally adding a downpipe with out an upgraded intercooler can be done but like @djfourmoney said, the heat soak can get to be a bit much. Also tunes can differ from place to place. One place my try to get max HP and Torque another can focus on drive-ability or usable power. Its up to your end goal of the car whether its a daily, auto cross, drag etc. Let the tuner know your goals and let them do their work.