OEM intakes and exhausts are engineering compromises they need to stay within certain cost constraints etc. OEMs build performance intakes like for my 2007 Tahoe, GM Performance has a factory approved cold air kit that improves performance and at the same time improves fuel economy. The reason people can have lower fuel economy with these types of improvements has to do with driver issues such as the tendency to use that additional power by more full throttle starts etc. If you use the freed up power wisely you will get increased fuel economy. I saw significant improvement with the GM cold air kit on my 2007 Tahoe both in fuel economy (when I keep my foot out of it) and power (when I need it such as when passing or towing). OEM cast exhausts are similar in that they are restrictive and intended to barely meet the performance targets set by the OEM. Long tube performance exhaust headers use tuning to reduce heat and backpressure to improve performance.
2004 Chevrolet Silverado 3500 LT with a 2004 Jayco JayFlight 29BHS
2014 Greyhawk 31FS with a 2007 Tahoe toad
2014 Thor Palazzo 33.3 with a 2007 Tahoe toad