Here is a test performed both on the dyno and on the street. I have used the Hypertech Power Programmer Plus Scan Module to record Mass Air Flow (MAF) measurements during a 3rd gear roll on. These were performed on the dyno with the hood up, hood down and on the street. As you can see, in 3rd on the street the car gets more Grams per Second of air than on the dyno. Below is a graph of dyno runs of the 3rd gear roll ons with the hood up versus the hood down. You can see the difference in horsepower on the dyno, now you could infer the added horsepower the car will produce on the street.
Here are the same tests performed in 4th gear. Here, from 3000 to 5000 rpm, there are some very large gains in air flow. I am under the impression that at lower rpm's, the engine does not require as much air, so the airbox is making a "Ram Air" effect, but as the rpm's go up, the air box cannot keep up with the demands of the engine. Below, I have again provided the dyno graphs of the difference between having the hood up versus down on the dyno. Again, it's a pretty big difference. There must be a huge difference on the street given these graphs.
Now here's 5th gear. In 5th the gains do not appear to be as significant. This I cannot explain other than maybe at these speeds the snorkels are no longer efficient at pulling in air. Just for your information, yes, the 5th gear on the street was measured up to 150 mph at just under 5300 rpm. The dyno comparison is below.