is your car an s1 auto, if it is,as no nistune, check boostcruising and sau classifieds for turbo (get your hi-flow checked as gcg is a great budget alternative), i would be thinking twice about getting rid of the ces dump as they are the best in the business,as far as the o2 sensor fouling a mounting bolt that does'nt seem right( are you sure it,s ces,. ces systems are stamped) ces rb25 front/dump have there 02 sensor underneath the dump and not next to turbo( not sure if older versions are any different, if not sure about dump pipe ring trevor at ces.
goodluck.