Gains as in an increase of available air into the intake, absolutely. I witnessed a considerable difference on the dyno with this set-up. (the pics were actually the morning of my 142whp dyno session.)
But as for building positive pressure into my intake manifold (boost), no dice. I never saw any sustainable positive air pressure on my vacuum gauge. And if there was, it was so minimal that I didn't see it. Positive boost pressure is less than 1% at hiway speeds. Had I more access to welding materials and more budget, I probably could have fabricated something a tad more useful and completely sealed.
Through all the research I did, the one I was most interested in was a motorcycle magazine test of 5 bikes which used ram air to boost their top end performance. Test were done on a dyno with the aid of some pretty big fans. Although the fans didnt provide the necessary air flow that simulates 100mph+ speeds on the open road, it was enough to test the ram air capabilities of their air boxes and see the gains as a result. It was around 110mph at which outside air ramming into their air boxes overcame the pumping forces /air speed of the pistons. But even then, gains were maybe 1-2%. Its not until speeds of 300mph does ram air actually matter. Rolls Royce discovered this in the 1920's. When they adjust fueling for their engine, they saw power increases of 15%. So realistically, ram-air on street cars: Myth. Ram air on race cars: 1, maybe 2% power increase depending on speed.
You will get more power gains by air box volume tuning. Ever wonder why our cars have more torque with the stock airbox than a CAI? Just look at the design; Built in ram horn (velocity stack) and the plastic ribs inside the box structure. All this is to increase air flow into the intake tube and control the resonant frequency of the air. Those ribs cancel out any frequencies at specific rpms to deliver a smooth, not peaky, torque curve. When people start 'shaving' the insides of their airboxes, I just laugh. Or when they remove the entire airbox and slap a cone filter right on the end of their MAF. Again, I laugh.