Is anyone using an APEXi AFC - if so anygood - I have a 1998 sti with cat back exhaust and ITG panel filter - what benefits would I get if any

 

The AFC is used to adjust the fuelling (particularly when increasing boost).

That would have as a benefit:
- that you do not run too lean (engine failure)
- that you do not run too rich (lack of power + increased fuel consumption)
- that the fuelling is right for the boost you run

You could increase boost by changing the restrictor in the hose to the wastegate actuator, for example.

Make sure your ECU will let you run the boost you like without cutting off the fuel.

 

The ECU also uses the MAF sensor as part of the algorithm for picking the point on the map for ignition timing. So if you try to lean out your mixture on full load the ECU will use more ignition advance. On an STi already mapped for 100-102 RON this could be too much - I suggest you use a Knocklink or det cans.