One last post and off to bed, it will be good getting back to work in a week or so LOL
A vacuum referenced regulator is generally used to overcome pounds of pressure that an injector sees under boost.
In other words, when at 20 PSI of boost, to get the same amount of fuel out of the injector as zero lbs of boost there needs to be and extra 20 psi of fuel pressure, assuming no loss in the system. This is of course due to the pressure in the intake manifold that the fuel in the injector has to overcome.
Naturally aspirated fuel pressures are far less critical, but indeed the OEM engines use them anyway. In this case they are accounting for inches of mercury in vacuum at idle, say 18, pulling fuel through the injector, then at full throttle, when vacuum is at zero the regulator has to increase pressure, but not a lot
In fact, the difference between 10-12 inches of mercury and zero at WOT on a naturally aspirated performance motor is a far cry from 10-12 inches of mercury at idle and 20 psi of boost at full throttle on a turbo or blower car. So on naturally aspirated motors most guys just let the computer adjust pulse width to make up for it. In fact going from 12 inches of vacuum to zero is a change of about 5 psi injector head pressure. The change in a turbo car could be over 25 psi depending on how much boost
As you pointed out, in Jason's case, ported vacuum would increase fuel pressure at idle, then as he accelerated, it'd drop off and then later increase as the throttle position and load changed and ported vacuum came back up. This makes a backwards and almost unrepeatable and unpredictable math problem for the ECM because it is looking for throttle position and vacuum from the engine but but the fuel pressure (which is unmeasured) is at times inversely changing compared to what it needs. Actually, vacuum is even shutting off at some throttle positions due to the ported vacuum, which equates to what the regulator would see when at WOT (even though it's at idle)
If it was mine, I'd run it unreferenced, and I do on my own, and mine is an EEC-IV computer that previously used a vacuum referenced regulator. However, if any owner wants to vacuum reference on a naturally aspirated EFI system, it's fine, but it really must be manifold vacuum. It is a more repeatable function that better matches with the TPS and MAP inputs to the ECM. On a turbo car, different story, I consider boost reference a requirement because it needs to overcome a wider range of head pressures
My 2 cents