I have an '05 Track and have driven two Nismo's. Definitely a noticeable performance gain in handling. That said, I would imagine the largest gain can be had by simply replacing the suspension:

Front:
Coil spring, 350z, coupe, w/nismo $79.16 (2)
Strut, 350z, left, w/nismo $153.31
Strut, 350z, right, w/nismo $153.31

Rear:
Shock absorber, 350z, all, w/nismo $111.20 (2)
Coil spring, 350z, coupe, all, w/nismo $133.51 (2)

Front sway bar appears to be the same, rear is different and says "call for price". Assuming you can reuse the mounts/boots & hardware my quick price search comes to about $950.00.

Also - as much as the wheels are "worth", they are absurdly heavy. The v1 Rays Track wheels are 18lbs a corner vs 26(?) for the Nismo rears. To me it would basically be a burden as I wouldn't be able to sell the OE wheels when I upgrade to lighter ones as selling the car without the Nismo wheels would kill the price.

Nice looking car but i have my doubts on price/performance here.

 

I second to this....i couldn't have put it better myself.