I'll stand by my statement that back in the day swapping the airbox for individual filters could often provide a useful power gain. Old airbox designs were for the most part designed with only a secure position for the filter in mind, and ease of access to the filter as well. The typical liter bike of the late '70's fed it's 1000+/- cc engine through a hole up under the seat about 2" in diameter.
Here's a shot of a page out of Bell's 4 stroke tuning book that shows, in his usual careful manner, a series of dyno tests that do in fact show some good hp gains from removing the airbox and using individual filters.
I'm not arguing your dyno results, DOHC, but I'd then wonder that if restricting airflow more than stock in some cases yields an increase, might that indicate a lean condition that was partially corrected by the reduction in airflow? Would rejetting those carbs and using the stock airbox possibly yield a gain?
One could argue that the power curve suffers in some way often by mods such as these, and that is true. The results in the picture illustrate that. Gain some at 8K, lose some at 4k. If that is acceptable really depends on the characteristics of the curve you are after. Modern high performance bikes take full advantage of airbox design, but that's tough to implement on the old dogs most of us are fooling around with here, some part of the frame is always in the way it seems.
It's been a long week at work and it's Friday night. Time for a beer!