A good method of determining if your intake flows enough to support your engine flow needs is to hook up a vacuum gauge in the plenum or intake manifold. If the gauge reads zero vacuum at maximum rpm, full throttle then there would be zero or very little gains from an aftermarket high flow intake.
I spliced a gauge line into the vacuum line on the manifold of my 2K2 Maxima and ran it to a vacuum gauge. See pics for gauge and installation.
http://www.fbody.com/members/LarryS/vacuumline.JPG
http://www.fbody.com/members/LarryS/vacuumgage.JPG
At full throttle, maximum rpm (about 6300) I read about 1/2" of Mercury of vacuum. This indicates a restriction. I removed the cold air intake box assembly from the box that holds the filter and ran again. This time zero vacuum. This indicates to me that the factory filter is sufficient to flow enough that adding a cone type filter would do nothing for hp with the cold air plastic assembly removed.
If you have an aftermarket exhaust then the factory filter may become restrictive. All my testing was with factory exhaust.
In summation, an aftermarket freer flowing intake with stock engine and exhaust will help but you can do the same thing by removing the cool air box. My next project is to modify the factory piece so I keep cool air coming in and get a zero reading on my vacuum gauge.