Difference between stock NISMO exhaust and FI exhaust is about 7-8hp. So in theory you will gain hp switching, but no much. However I did this swap and it was noticable for the butt dyno to feel but not so much that it felt like a new car or anything.
All that said, considering you already did the intake mod I doubt you will see much gain changing exhausts. The gains from individual mods don't add up. Meaning that if intake gives you 15hp and exhaust gives you 15hp it doesn't mean you end up with 30hp. As you start adding mods the gains become less. On a normal Z intake and exhaust tend to give a 20-25hp gain. Considering yours is a NISMO that already has an upgraded exhaust and you've now added intake I would say switching to FI exhaust probably will only yeild 5hp, maybe less. The sound though is worth it so I say go for it.
The above poster got a 40hp gain but he also has HFC and was tunned. Something to consider if intake and exhaust aren't enough.