If i'm not mistaken I think the highest factory gear ratio Nissan put in the r200 was 3.90's. You might check with zcarparts.com if you end up looking for diffent pinion sets for the r200. From my experience with them on several z-cars they can hold up to a lot of power and torque. The design on some of the ones in the 280zx's were a little odd in the way they mounted. I had 5 z-cars before buying the S.
I've seen one of the rear diffs form a 280zx hold up to a small block Chevy pushing out 750+hp. They are good diffs. Do you still use the rear mount that comes off of the diff cover? I had to replace a few of those on my Z's (damn rubber deteriorated). I was wondering about the setup because I think I have one sitting around sitll and was considering putting it in before goign FI.
Weren't the housings on the r180 and the r200 virtually identical. The R200 was used Z cars as early as the 260 2+2's. the r180 differential if i'm not mistaken was almost identical to the r200. Nissan Differential information
I used to own several z's. My '81 2+2 had a r200 with 3.91 rear gears. The turbo 280's had much lower gear ratios.