Now your speaking my language.
Fastchip makes a ECM Prom for our 4.9L engines. It advances timing to 14-15 degrees BTDC and eliminates the top speed limiter (which is 112mph btw)
HP Yield= 10-20bhp, depending on the health of your ignition system.
The 4.9 already has a cold air intake, the problem is that the intake mouth is only about an inch wide.
Evoluzione Engineering makes a 3'' Intake for 4.9's, as well as a few other performance parts for Deville.
HP Yield: 15-20bhp
Your pretty limited on exhaust, unless your able to do custom work. The biggest improvement you can make is higher flowing muffler and cat. Really any muffler that's offset in/center out will work.
HP Yield:10-20bhp, depending on what kind of muffler you get. '
There's a few more non-invasive things you can do, but I'm hungry and don't feel like typing now.
These are your biggest non-invasive performance mods though.
Disclaimer: HP numbers are estimates, and your actual numbers may vary. Note that this is also HP at the flywheel and not at the wheels.