New exhaust yes or no.

Well we just completed ours on a 6.0 07 Chevy crew cab. A few weeks ago we installed Banks headers since we had a broken exhaust stud I saw no reason to reinstall the cast iron manifolds. We then towed our enclosed trailer and picked up about 1 mile a gallon or so. Performance wise, it felt slightly better, maybe a few HP. I knew the headers could only do so much with the stock exhaust system and unlike the comments above the stock cat back exhaust was fairly restrictive on my truck. So yesterday we installed a cat back Gibson single exhaust. Although we haven't towed with it the motor has come alive and revs much free'er then it had. Almost feels like and overhead cam motor now. It is a little noisy in the 1200-2300 rpm when accelerating, once at cruising speed it sounded close to stock. I've raced enough of street cars to know that the best way to increase HP is to dump the stock exhaust. Even when I tracked Porsche's the exhaust was the first place to gain HP, and they have very good exhaust systems from the factory, far better then what was on my truck. We'll see how this translates into MPG when towing my trailer and if it was worth the $$ or not.