It's probably not worth getting all worked up about the Nikasil issue at this point. If the engine is running well, as yours seems to be, your car has dodged the bullet. The way to keep it that way is to make sure you use low sulfur gas. Good luck however trying to determine how much sulfur is in the gas available to you with the post Katrina revenue enhancement ^w^w formulation requirements relaxation program. There was a BMW program to replace nikasil engines with alusil but that's long since expired.

As far as a chip goes, there's not a whole lot of options there. The engine is already running close to optimum so there's not a lot of room for electronic tweaks. Do NOT mess with the airbox. Not one of the aftermarket suppliers of "cold air intakes" (which you already have) has ever shown certified, independent verification of any power increase.

IMO, if you'd like a bit more exhaust note, that's a reasonable thing to tackle.

You might want to change out the cabin air filter. Search for instructions on how to do that -- it's a bit of work. Chances are that yours is cruddy.