I don't know about 2000+, but gutting the box and putting in a K&N improved my '95 SE.
A year after doing the PBC "best practices" tune-up, I did mine. The MAF and manifold readings changed immediately. I didn't know if this was a good thing, more air means more gas, and it was my DD. I had been skeptical, but I recorded some data, and gave it awhile. I wanted to see what the PCM thought about it, if anything. After I had done the tune-up, the whole car had changed significantly, and IMHO that still remains the most return you can get for your car. So, everything had been stable for quite a while, and it would be a good test.
A month later (with a few peeks in between), I broke out the scanner. The most obvious change was the LTFT. But it didn't move up like I speculated (I didn't understand how the different cells add to the LTFT at the time), it moved down closer to 0.0 for its predominant static state.The second obvious change was how fast the O2 sensor reading moved from LEAN to RICH. It hopped like bunny. This would flesh out in improved readings in my next emissions sniff.
The 90 days after the change my mpgs improved by 1.8, The next 90 days by 2.0 mpg above the pre-test mileage. (All measures seasonally adjusted) Which is not to contend a specific nominal improvement, particularly at 8-9%, that would be ridiculous. But it did unquestionably confirm there was some improvement in mpgs.
Did I have more hp? I have no freakin' idea, I didn't measure it. But at idle, the unnoticeable vibration in the steering wheel disappeared to a dead calm. My wife noticed it immediately, when she stole the car for a day in the midst of the test.
(She can be a real nitpicky biyatch with her '96 Majestic Teal. She has incredible sense and feel with her car. She sensed dysfunction with her car a month or so before the anti-freeze started to visibly disappear. Our mechanic, who owns a '95 SE, and I couldn't convince her. Besides checking all of the other usual suspects, no chocolatey oil, no sweet exhaust and we both drove it. There was a slight cleaning of two plugs, but insignificant enough to be random. She didn't know, or care, that it was a UIM/LIM situation. She just knew her car "wasn't right". And she wanted one of us to fix it. )
Was the throttle peppier in the low and midrange? I definitely thought so, buts it'* equal money that that could be real or imagined. I will be real stubborn though, about the 3rd gear to lock shift, up an incline. It was no longer so flat.
So can just increased aggregate air supply, increase aggregate horsepower? Well, unless you can further compress it into the chamber, and have the appropriate fuel and ignition advance waiting for it, I think you have a real uphill battle against some basic laws of physics.
However, if you can maximize specific narrow situations where the NA stock has demand points which are adjusted for, rather than properly supplied by the stock kit, then you have small opportunities for more effective utilization and efficiency. For which, it appears to me, the PCM is happy to accommodate.
My experience and measurements on my '95 and '96, indicate that these situations do exist on the 3800. And added together make for more efficient operation of the car when you take into account the entire spectrum of performance... and enjoyment.
1995 SE ..Green and Rattle Can...234K mi...Bonnie (she'll outlive me)
1998 SSEI .. White ..119K mi...Evey
1996 SSE ..MajTeal..157K mi...4/12- Dealer trade full of Lucas Trans fix