The plastic diffuser is really easy to cut with an exact o knife as well. It's a lot easier if you remove it first.
There are 4 plastic retainers, 2 screws and the rest just clips on, you can remove it in less that 5 minutes.
Take your time and just score the plastic with a sharp knife, going over the line a little deeper each time, after 3 or 4 times you"ll be able to cut through with the knife leaving a smoother line.
To finish it off nicely, get yourself some door edge trim, the kind you push on the edge of the doors to stop them getting dinged up if you open the door into something, and use a heat gun/hair dryer to soften it and make the going around the sharper corners easier.
Check out the post on my Dual Exhaust conversion:
http://www.forteforums.com/forums/body-kits-appearance/6698-dual-exhaust-sedan-sx-pictures.html