A friend of mine just got an 06 6.0. He went to get a 4" exhaust for it and wanted to remove the cat and straight pipe it. He didn't cause he was told by the shop exhaust shop that The computer was programed for the stock exhaust and if he would to up grade it would screw up the PCM and make the truck run worse. Is this true? I always thought that if t breaths better it runs better. Any advise would help thanks!