From what ive read the 6.4L intake does produce a little more top end horsepower because of the Short Runner Valve but not substantial enough for me to make a difference as my long tubes probably pick up what was dropped from the intake if any. I mainly chose the 6.1 because it bolts up without modification and I wanted the strength of the aluminum over the plastic as i plan to add a procharger later on. Not sure on the aftermarket intakes. I know edelbrock makes the VictorII which resembles the 6.1 factory intake, probably flows a little better but im not sure. And the only other aftermarket intake ive seen that i liked was the Plazmaman Intake but those are very pricey. They look real nice but im not aware of how they fit in swaps.