Just been seeing a lot about them lately. Are they a part that really adds power or more of a way of giving "potential" for power?

 

they add power, but also add more when supporting other mods as well. the fast intake is a tuned version of the LS series intake. the number usually stands for the mm of TB it will accept. so a FAST92 is for a 92 mm Throttle Body and so on. they seem to be the most expensive but yeild the most power in LS motors. its overkill in an LS4 if you ask me. the 5.3 doesnt need to breath that much and you can achieve the same with a LS2 Manifold and TB and spend hundreds less.

 

All of the above. The ls4 manifolds chokes the crap out of our motor

 

Couldn't have said it better myself. The only intake restriction the LS4 has is the intake manifold itself. And at that point a ported LS2 or a LS6 intake manifold will be cheaper and make the same power.