Another thing you have to think about, is that on a dyno is a lot different to when you are actually travelling at speed.
I mean in a real life situation you may have placed your intake in a high pressure zone, or somewhere with good airflow. But this cant be replicated exactly on a dyno. Even if there is a big fan blowing, its not the same as the effect you would get if the car was actually travelling at that speed.
But... yes that seems strangely low regardless. Even for a 20v