Iron manifolds have been used many times on LSX turbos over 1,000 hp.
IMHO unless it's really important, and you understand and accept the bad things about tube headers (will only last 1-2 years, huge increase in underhood temperature, loses much heat energy that the turbine will never see, slower to spool up) in exchange for better exhaust flow.
I had the factory iron manifolds on my Toyota turbo to 520 hp (very common), using a SPA (Brazil?) aftermarket cast iron manifold with Swain coating for the new 700 hp engine this year.