Well...since I wrote that article, I should respond. I thought that it definitely made a difference in pull across the entire RPM range. Obviously with the bottom cut out, at higher RPMs it starts to breathe a lot better than through that tiny hole in the front of the factory airbox. As for power made, I still need to post my dyno numbers but, here's my original post
http://www.lincolnvscadillac.com/showthread.php?t=6330 .
The biggest benefit to this free mod over an open element in the engine compartment is that it will still suck cooler air from behind the headlight/foglight. Some of the others you'll notice are an open element in the engine compartment. What this will do is suck in hot air...not good for air density in the intake tract. Someone did a compare of the temps with a protective box around the open element and without. From what I remember, there was quite a temp difference. That said, the modded air box will sheild from the engine heat.
One thing that was discussed in depth was the element size of the flat panel vs. a cone. I don't know that it was ever proven that the cone provides a huge difference over the panel on the 3.9L.
As usual, my $.02.