Would it be worth the investment to install an Apexi Fuel/Air mixture control onto an '86 n/a?

 

All factory mixtures, even NA's, are very conservative for reliablity reasons. At full load the engine runs quite rich, so there is definitely some scope for finding more power by leaning out the mixtures, while still remaining safe from detonation.
The more modified an engine is the more likely the mixtures are less than ideal, so it's a good investment if you have future plans.
This sort of tuning should only be done on a dyno so mixtures can be tuned properly with a wide-band A/F meter.

 

What model would I be looking at buying?
I'm also looking at buying a fuel pressure regulator for higher fuel pressure, I'm buying a TII fuel pump along with running a 12V system instead of 10V. Any idea's?

 

S-AFC What are your mods? Even fairly modified NA's run fine on the stock pump and regulator. There's no need to replace either until you need more fuel than they can supply. The fuel pump already runs on 12V, not 10V. If you thinking about the relay/resistor that drops fuel pump voltage at light loads, S4 NA's don't have it.