[DeleriousZ]

First off, welcome to AF! Secondly... glad that you came over from the dark side (you'll find most of the people around here aren't mustang fans.)
alright.. down to the good stuff... i wouldn't bother going with the k*n fipk... you'll just be paying $150 for something that would work just as well with a generic 9" cone filter. it's the same prettymuch with any brand name stuff for the z31.. a custom 3" mandrel bent exhaust system would probably cost the same as the borla.. perhaps a little less, it's good to do some research on it before you jump into anything. 12 psi is going to be pushing that thing pretty hard with no intercooler.. you might want to look into getting either an intercooler or some sort of water/alky injection system. if you don't want to bother with that stuff (for now) the best bet would be to set it at 8-10 psi and maybe jump to 12 if it's a cold night and you need a momentary boost of extra power.

http://z31.com/tfaq/ is a good page for what you're looking for... z31.com has some good information on it for other stuff as well. also... read the stickies at the top of the page, they're filled with good info.
hope you enjoy your time at af