Are their any performance gains from going to 496 manifolds from stock iron merc 454 manifolds besides just the weight savings.

Nope still a log manifold. I know raylar has used them a lot but i would spend the extra money and get some stainless marine mani's. I thought the 496 manifolds were still cast iron it was just the risers. They do look better tho.

actually there is a gain to be had when switching from the 454/502 manifolds to the 496, these manifolds actually flow pretty well and i beleive were designed by gill who made the exhaust on the older hp500's, the difference from the 496 to the stainless would be very minimal and not offset the cost difference, and yes the manifolds are cast iron but were also available in aluminum in some years with stainless risers, if your looking to gain some power without spending $2500, then this would be a good choice, if your going to spend the money for stainless marine and boat in fresh water i would hands down go for the dana's, they will perform very close to any tube style header without the worries of cracking, and cost comparable to manifold style exhaust, bang for your buck they are number 1 imo

Dry joint is a plus too on those manifolds.