Air filter won't do much. K&Ns don't tend to increase performance that much, your best bet, which won't be much performance increase at all, is a cone filter that is filtered at the top and not just the sides or a better drop-in.
If you were thinking of a new air-intake system as opposed to an air filter, then expect some performance increase. Tim Taylor (The Tool Man) has a custom Deville with an awesome new cold-air intake. That thing just cruises.
As for pips, it depends on what year. You could buy a completely new aftermarket exhaust system if it's 98+ or just replace the current pipes by increasing the size and adding performance mufflers.
It all depends.
