Ive always used mild steel manifolds. As i couldnt be bothered with a stainless cracking all the time. A lot of people run stainless ones without crack issues though so i think its just down to personal preference.
Yeah main reason most people run them on CT9s is too prevent boost creeping issues. And also for the noise. Ive been externally gated for about a year now and i wouldnt go back to internal gate anytime soon.