The restriction starts at the manifold, but yeah, that pipe is restrictive. If you have 10 feet of 1 1/2 inch pipe then you put on a giant *** muffler with a 6 inch opening, it doesn't really help as much as having bigger header pipes replace the manifold and all the pipes all the way back. The reason the manufacturer puts this pipes on is for different reasons. Pollution control, noise control, or cost of materials just name a few.