SSTAHOE6.0LQ9

once you start making some more power you will wish you had some decent headers just because of the fact that headers will remove the exhaust gasses must faster and more efficiently than any stock manifold period.
Stock manifolds are designed for low cost and have a short restrictive design
which will actually promote turbulence which causes exhaust gas to get bottled up in the cylinder heads. Because of this residual exhaust gas in the cylinder heads it limits the amount of the new fresh air fuel charge is and will increase the exhaust temperatures. Any decent set of headers will have smoother individual pipes for each cylinder, which pulls the exhaust gasses out quicker creating a suction effect called scavenging, which pulls exhaust out. This creates additional velocity in the cylinder head and helps to pull in the new fresh air fuel charge.