Remember that in addition to that stuff, you need, intercooler(if not included), Fuel Pump, Injectors, ECU, somebody to re-map it, so still on top of that, there is still a large amount of money that needs to be spent.
And you can also put that stuff together for a fair amount cheaper.
The manifold design would be more important, however, steel is generally weaker and more prone to cracking than cast iron, however it's easier to work with for making tubular custom manifolds so T304 stainless is often used.
Though the manifold design isn't that important on forced induction on a road car, obviously it's important on a race car and will make a little difference.