I'm sticking with my previous post and honest butt dyno results... cold air intake (large one, preferably K&N) matched to a good exhaust (I picked HKS but when I saw the size/weight I was not exactly pleased). The COMBINATION of these two things with nothing else made a noticeable increase in horsepower and I noted no real losses in the low rpm range (and the car idles quiet but sounds Wicked Mean from 4,300 rpms and up... I can link you to a video where you can hear the car wind out as my camera man videos from out the sunroof if you'd like... with mk2's falling behind as I pull away). Top end definately winds out better and I rarely cruise under 4000 rpm. These 4ag motors were made to rev high, they seem to prefer it actually. Of course doing just exhaust will do nothing because the motor is still restricted at the other end (duh)... do both (without going overboard on the exhaust end), and you'll notice an increase.
Just running a header and downpipe would be a hefty loss in power more than likely due to the extreme loss of backpressure... gotta have some backpressure in these small, lightly modded motors folks!
Oh, and here's the
where you can hear the N/A... sorry for the bouncy camera work, a broken strut and hard driving with someone hanging out the sunroof trying to film is tougher than it looks! I also couldn't see behind me out of anything other than side mirrors so I had no clue I was screwing up the video pulling away from the other cars. You can hear the N/A really good from about the halfway point on when I get in front of the pack and lead for a bit.