A velocity stack is basicallu a bernouli bell mouth, it's designed to allow the flow of air to pass from an infinately large reservoir into a circular opening while maintaining laminar flow characteristics. This allows for a large flow of air to pass through the opening, increasing velocity, while maintaining laminar (smooth) flow of air. In our case, you would get a smooth flow of air into the intake manifold, and then you'd destroy that flow of air as it's broken into 8 separate streams that are pulled through the intake runners.
Put this in the same category as throttle body spacers. People believe a throttle body spacer does something special by swirling the air as it passes into the intak manifold... and then it's destroyed as it breaks into the 8 intake runners. The only place you see any benefit with velocity stacks is if you are running turbos and don't want a filter, or if you are running 2x 4bbl carbs and you have a stack for each barrel of the carb.