While the factory system on its own is not bad, it still is not as good as it can be.. The DMP exhaust has long internal runners, and has runners that carry into the risers. There after exiting thru a venturi they call a vortex. The venturi helps to speed up the exiting exhaust more than many collectors in headers. This also helps to draw in more air fuel mixture into the engine. This requires an adjustable fuel pressure regulator to enrich the fuel delivery slightly. That's how the power is gained with this system.
In reference to Ray @ Raylar he has dynoed the engines with these kits & he has substantiated these gains & the need for the fuel pressure regulator on the Mag Ho applications with the modular fuel pump system.. We are a dealer/installer for his products as well. I have personally talked to him about these systems. Unless I misunderstood Ray, then even though many of the engines you see in his ads have CMI, he said that the DMP systems also compliment his products nicley..
BTW: Ray has if my memory serves me correctly said that the factory exhaust was good for a merc stock part. However Ray himself was able to improve upon it & used to offer that service at one time. He felt that the DMP was superior to doing that, especially with the manifolds going back to cast iron.
As far as the cheapest bangs for the buck, this is one of them..
I hope this helps to shed some more light on the subject for you guys.
Jamie / Lakeside