I did notice a difference going from my ebay 4-1 with 2.5" collector to a 4-2-1 2" collector. I did however cut most of the collector off and welded on a 2.5" piece but I still feel like I lost some top end power...but on the plus side it seems like my power band is much smooth and more drivable with my modified 4-2-1 with 2.5" collector.