[coyoteX]

I am running an AFR gauge also, it lets you know what the computer is seeing in the exhaust. The Vacuum gauge watches the intake side. The AFR gauge watches the exhaust side. Being a Tech, you know the Intake Event is the most critical. Read some of Bisimotos stuff, the Honda single cam master. When climbing hills, I try to keep the AFR pulsing with the best possible Vacuum. Usually only 1"-2", on the Vacuum gauge. When the AFR pegs out rich, that's unburned fuel out the tailpipe. Letting off the throttle, ever so slightly, gets it pulsing again. On downhills, If you crack the throttle, you can actually pull more vacuum and see no pulse or very low AFR reading. I use a ScanGauge II on my '01 Tacoma and it's cool to watch the MPG go to 9999. When you get the technique down, you can actually let off the throttle slightly as you are cresting hills, watch the Vacuun actually increase and be accelerating at the same time. This is completely contrary to how most people drive.
As far as good engine mods, a 4-2-1 header is necessary, so the exhaust pulses hit the main pipe in sequence. As far as exhaust back pressure and harmonics, I did the math and Honda already has that taken care of. A full exhaust, header to tailpipe tip the stock size would be ideal, but you won't be able to come up with a small tube header. It's not a problem though, because as long as you go back to the stock pipe size behind the header, you regain exhaust speed. Big tubes are only good for astronomical RPM's, racing only. No good in the real world. Hey, you can always sell it to Ricky Racer.
Good luck Dude.