I have actually purchased that same set of rockers for my engine, but I'm going to be running the factory camshaft. Haven't gotten them installed yet, but am hoping to do so soon!
Keeping it as simple as possible, the larger ratio (vs the factory 1.6) will slightly increase the valve lift, which will allow the cylinders to take in more air/fuel.
That said, if you're swapping in an aftermarket camshaft, you likely won't need them, and from what I've read, it's generally considered a better overall route to swap to a "bigger" camshaft than to swap in larger ratio rockers, but it doesn't take much to have to switch from "canned" tunes to "custom" tunes, and the price starts accelerating quickly from there. For those of us looking for a boost to airflow that can be handled by "canned" tunes for our Magnums, the 1.7 rockers work well.
That cam you've picked out sounds interesting; lots of duration with a narrow LSA. Don't suppose you could provide a part number?
