Power gains would most likely be cancelled out, but seeing as you have an '02 JDM, I may well be wrong, because there has been mentions that the early JDM's ECUs don't cancel out the mods.
If you do manifold, y-pipe, decats and the K1, then you'd might want to invest in some eardefenders too, lol.
The K2 will be quite a bit quieter I'd say.