I havn't painted anything under the hood on my CTS, but a word of advise from my experience with other vehicles is to sand all the plastic parts as smooth as possible to get rid of any texture they might have. That way when the paint is applied & dries, it's smooth and glossy. It looks much better than painting it in it's original condition. Be sure to clean everything really well to remove any dirt, oils & other debris. Dish soap works great.
Here's a before & after of a couple pieces I painted under the hood of my Camaro. The before shot doesn't look that bad in the pic, but in real life both pieces were dull grey plastic & looked like Ass! It's a bit dirty in the after shot, but ya get the idea.
