question...how do stainless marine 5inch generation 3 exhaust manifolds do with 960horse 555c.i. procharged engines? do they loose any power? thanks

That is exactly what I`m running on my boat. They are way overkill for my little 489 cid (about 525 hp). I got a screamin deal on them , so decided to get them anyway.
From what SM says, they are good to 600ci. If you were to look over a set of these, you`ll see that they are huge.
I would say that these are about as good as anything on the market for your power and cubic inches.