My son just got the Autocross bug. He drives a 2006 Impreza, an wants to do a small upgrade to his sway bar. One of the forums he is reading says to just upgrade the rear to a larger dia bar and leave the from sway bar stock? Any insight on this?

That will certainly help make the car more neutral. Probably the best thing he can do to help his autox times apart from tires.

I did the same thing with my A4. Granted I have Koni coil overs on it but I upgraded to the largest rear I could find, threw decent tires on it and it became the best handling car i've ever driven.

Make sure the rear sub frame/sway bar brace is strong.

This will all allow the car to have a nice lift throttle oversteer which is awesome for autox.

http://www.youtube.com/watch?v=Zl3N4G88DM8