PSB,
Whether you need an extension or not depends on where you want to mount your screen. The center line of the projector lens needs to be about even with the top of the screen. One approach is to experiment using the Chief mount and making up extensions of various lengths with PVC pipe. 10 ft of schedule 40 PVC is about $3, plus a few more bucks for fittings and PVC adhesive. Takes only about 15 minutes to build 3 or 4 different extensions. Once you have one you like, take it to Home Depot and have them cut you a steel version ($3/ft, threading is free).
If you get a Chief mount, don't forget you may need to get a ceiling mounting plate too.