The people from the Maxima forum said the total piston area for 300zx is 3.94
I have no clue if it came from stock 300zx rotor or 12.6 rotor or it even matter.

 

3.94 sounds about right. I have not pulled the brakes off the maxima since the last brake job about 6 months ago. But looking online it appears to be what they say on the org. Mine is the 13" 300zx kit not the 12.6 but that doesn't matter as that is just the rotor size.

 

That front piston area is certainly close enough to the Infiniti piston area, I wouldn't be concerned w/ the biasing.

What rotor are you going to use? Just be sure the rotor width (not diameter) is within a coupe mm of the original Zs thickness, obviously too thick a rotor won't work but one that's too thin will cause too much piston to extend which will result in possible seal issues and extra corrosion on the piston sides.

Here's a pic of my Wilwood clearance, these have been on for 10k miles w/ no issues and a very aggressive bed in where the rotors were scorching and if there was any expansion of the caliper body it wasn't visible to the naked eye.



And my hillbilly rolling shot

 

for my setup I was using the 13" 2000 mustang cobra rotors for thickness, with new pads I had about 1mm of clearance from the pad to the rotor. so going between ours and the mustangs you should be good to go.

 

At least Infinit has a couple different rotor thickness to choose from, you have the 13" base coupe rotors at 1.25" thick along w/ the big 14" sport rotors that have the same 1.25" thickness and the smaller base sedan front rotors at 1.1".
As far as having a smaller piston area than OEM, that will give you a bit more brake pedal, but probably not even noticeable, much better than having less pedal.

 

Wonder if I could use my stock sedan rotor. The 300zx calipers need to be trimmed down regardless to commidate the new rotors thickness

 

What was the thickness of the Z rotors? Not sure what you mean about trimming the caliper, do you mean the pad(s)

 

300zx stock rotors are 30mm thick but they are 11" rotors so the rotor channel on calipers need to be trimmed tiny or they will hit the 13" rotors. I don't know if that's necessary but that has been the way Maxima and S14 guys done the swap.