I just did this too. Seat dyno says noticeable improvement, especially pulling in first through second gear. Sound is pleasant and helps with shifting (not so quiet).
As for why it works, I am not so sure that removing the ram air scoop is the reason. Like you, I thought the ram scoop looked extremely restrictive. I was curious as to actually how small it actually was in comparison to the rest of the intake system. I am no engineer, but I measured the cross-sectional area of the stock ram scoop, the connection between the ram and the air box, and the intake hose after the air cleaner, yielding the approximate values:
ram scoop: 10 inches X .75 inches=7.5 square inches (s.i.)
ram to airbox connection: irregular shape yielding approx. 10 s.i.
intake hose after air cleaner: 3 inches diam, yielding approx. 7.1 s.i (area=3.14 x radius squared)
Disregarding changes in direction, I believe that the flow is limited by the smallest cross sectional area within the system (ie the most restrictive section). Conversely, does it not also follow that one can not net additional flow by increasing cross sectional area in one area over and above the smallest section? Following this logic, the bottleneck in the present system is in fact the 3 inch intake hose and the primary reason for the performance gains achieved with 2002Altima's ingenious modification might be the blocking of the resonator and to a lesser extent, restriction in flow from the sharp turn in the ram air scoop.
Personally, I am going to try hooking the scoop back up to see if there is any noticeable difference (also block the little resonator connected to the scoop). I think also that the scoop can be modified to double the cross sectional area over stock just by cutting out part of it with a Dremel/Rotozip. This might help negate the affect of the sharp turn in the scoop.
Wish I had easy assess to a dyno since this is really the only way we can tell for sure. However, I am going to try a simple gas mileage comparison.
