Also, in all sincerity, I think if you picked up a "take off" Nismo (the one with the H-pipe) at a decent price, you aren't going see much more springing for a different cat-back. Really to pick up more you'd need either headers or at least high flow cats. With either of those you will DEFINITELY need a tune.
With your current mods and OEM cats, a tune might pick some more power, and you could improve throttle response (and shift speed if AT), but otherwise I'm sure you are running safely without a tune at present.
It's hard to say exactly, but probably a solid 10 -15 whp or more over stock at various points in the rev range on a dynojet with SAE correction.
If you search through my thread with all the dynos (search for "proven power dyno" and my username) or look over the dynos in my gallery (most are in "Back in Black") you should be able to get a good idea as several of the dynos are of those or similar mods before and after tuning.
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