I was told that the Mercury Manifold risers are the "bottle neck" of the stock exhaust system. I am running a 454Mag (stock 365HP) with a B&M #90767 camshaft and a 177weiand with a 5lb pulley. I have been told that the stock exhaust is good for 450HP but with these risers it is good for 550HP. I don't want to spend the money for a Full exhaust system and will never build the motor for over 550HP.

Will this help performance?

I hope that somebody reads this...I would love to know as well.

I think if you're talking about the stock Merc exhaust manifold/riser, I think the whole system is the problem.