One thing I have noticed with just running a pipe on a stock engine is there is gain every where.

The right pipe adds to the bottom the mid range and the top rpm.
When the engine is ported the bottom end power goes away and the mid and top end are enhanced. Now I know this isn't always the case but usually it's true.

The longer the stinger the higher the operating range.
Low end comes from short fat stingers

The stinger is the pipe that the exhaust actually exits.