Depends on how much boost and power you want to make. If 300hp is your goal and you are running stock cams then a cast manifold will work very well for you. If you are running big cams you need an equal length long runner manifold. My personal car is going to make in the 350 to 400hp range and I'm going to run a cast manifold for awhile despite the fact I could build an equal length if I wanted to. Match the manifold to your setup.
This is true but people usually end up going bigger once they are tired of 200whp - 300whp
Then they end up using a new exhaust manifold, turbo, etc.