You have two areas to improve horsepower. First is the air intake. Make it less restrictive and bring in cold dense air. Its referred to as CAI or cold air intake. The TonyA system Hoth mentioned is the cheapest but it does work. I did a write up in the Cadillac FAQ section.
http://www.cadillacfaq.com/faq/answe...ram/index.html
From about $40 you jump up to the most popular CAI, the Volant at about $245. I started with the TonyA but later moved to the Volant. Both add a nice robust low pitched growl to the engine. That's about all you can do on the intake side.
The exhaust system is next. The cheapest route is adding a used CTS-V exhaust, about $125 installed. Horsepower gains are modest but it has a nice sound and magnificent chrome tips. The three aftermarket exhausts, the Corsa, Borla and B&B. If you have the 3.2 the Borla is the best choice.
I have the Corsa on my 3.6VVT engine.
Next is a big jump in cost. It's headers and high flow Cats. Only one company makes them, Cadillac MotorSports. I have a set on order @ $1700.
This is not a DIY so allow 4-5 hours of Pro-tech time.
The above mods should get you close to 300HP. There are some additional mods available. Very pricey and it challenges the integrity of our differentials.