| Spinoli1's comments about smooth airflow into the airbox reminds me of an additional issue.
An added benefit of using an airbox over the throttle bodies is that it provides a consistant, low velocity, less turbulent air supply that the velocity stacks can draw from.
The purpose of the velocity stacks is to take this air and create a smooth laminar air flow to maximize flow rate into the throttle bodies and intake tracts beyond.
You should keep everything away from the top of these velocity stacks because it doesn't take much to disturb their flow. For example, just placing your finger near the top of one of the stacks will show up as an immediate power loss on a dyno run. Another reason for avoiding over-the-bellmouth filters, it would seem. | 
Shazaam!
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