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The only reason id replace them would be because they were shot from thousands of heat cycles or I got a good deal on a set. The factory manifolds are a very good design for a turbo engine and not a limitation. The exhaust housing is the limitation because large turbines can't be optimized in it. I wouldn't expect any power increases in even an 800hp application by switching to any other header as long as the stock flanges were made to accommodate larger ex ports. The exhaust pressure vs turbine shaft speed is what needs to be optimized. You want to have the highest mass flow with the least ex pressure. Which means just enough energy to drive the turbine to keep up with the engines needs and no extra. There are things that can be done to gain an advantage with this if the compressor and turbine are working together at the right pressures. It's very possible to see a decrease in ex pressure while increasing manifold pressure all while increasing mass flow. You will find that the engines brake specific fuel consumption improves. You are using the same or less fuel to make the same amount of power. Pumping losses are reduced and the engine is more efficient.
