As far as better flow, the advantage would be minimum because of the "forced" part of forced induction. Your turbo(s) (or SC, whatever) will flow a certain amount of air into the engine regardless of how 'easily' the intake manifold allows it to go in. The mass flow rate of air ultimately going into all the cylinders will be equal to the mass flow rate your turbo pushes out. Now what will happen though is that the boost will go up with an unported manifold (or down with a ported version- however you want to look at it), but the same amount of air will get into the cylinder either way. Think of it kind of like a garden hose flowing into a bucket. Unrestricted, it flows out easily and if you leave the hose there in the bucket for say 2 minutes, it fills the bucket up at a relatively low pressure in the hose. Now pretend you put your thumb half way over the hose- the pressure definitely goes up, but that same bucket will still be full in the same 2 minutes, it just was under more pressure to flow the same amount. I hope this is making sense here. The same applies running ported heads. Except ported heads have an advantage in that the exhaust can be ported which will allow the air to escape better. The turbo forces the same amount of air in (in theory anyways) regardless of the 'ease' in which it happens, but you need all the help you can get to get the spent exhaust gases out.

Now having explained that, I believe the FAST intake manifold is also physically weaker than the LS6 manifold, and can blow apart under boost.