2012 Silverado reduced power issue

[Bradley William Bumbie Babarovich]

I have a 2012 Silverado 2500HD with a reduced power problem .throttle body performance  problem.The local dealership said I need to put the stock air cleaner back on to fix the problem but why for 4 years it ran fine with the cold air intake I had on it .my muffle sounds different and was curious if it could be a back pressure problem any thought or similar problems