The concept of altering AFM voltage is basically what the super afc does. While it may work well on certain cars with different EFI systems, its a terrible idea on a 280 or 300zx efi computer. The efi computer uses AFM voltage as a major part of the load calculation. By reducing this voltage you effectively reduce the calculated load value that the computer is using to choose the correct cell on the fuel/timing maps. Now this can lead to nothing or it can lead to disaster. It all depends on which cell is selected with the new voltage. For instance, suppose you have an AFM output of 3.5 volts while under full load. now you reduce that voltage to 3 volts, the load scales in the maps shift backwards (left). Take a look at this picture..
This is a stock timing map from a z31 (i forget which year exactly).. The rpm is obviously vertical, and the top numbers represent engine load. This is a calculated value. If you are interested in the exact formulas its described very well here: http://ztechz.net/id10.html
The cell values are actual degrees of ignition timing, lets look at the cell that corresponds to a load of 80, and 4400 rpm.. At this point you are in the middle of a full throttle run. If you were to drop the afm voltage then you would effectively moved the calculated cell to the left. Suppose the voltage drop at the afm shifted you 3 cells over, look at the timing you would be at.. You jump from 24 to 35 degrees.
Regarding the change in air fuel ratio, take a look at a stock fuel map:
As you can see, the same thing applies here. A reduction in voltage does not always correlate to a richer AFR.
I used to work on a kids r32 skyline with an RB26. He had more money than brains and decided to try and make the car fast. He took your idea to the extreem and used a safc to "compensate" for 720cc injectors. It took me an hour of math to explain to him that because of the changes he made he was running 14-1 AFR's and over 40 degrees of advance which resulted in the holes in his pistons.
This is an extreem example. He was basically cutting the afm voltage in half and screwing everything up. Small changes to AFM voltage that only shift the load scal by one cell will generally not hurt anything. It also will not help anything as it is a completly unpredictable alteration.
Your best bet is to spend the 400 bucks on a nistune daughterboard (need to have a z31 ecu conversion). This will give you full control over the stock ecu with complete predictability and repeatability.