I've got some hoosier autox tires mounted on 17x7.5 m3 forged wheels. Should I have them balanced? Will it affect performance if I don't? One wheel is about 5 oz off.

For auto-x, i personally don't think it's necessary to balance, since the speeds aren't that high, and not sustained for very long. I ran 17x9.5's with 255/40 Victoracers on my auto-x Mustang for several years without balancing them and never had any vibrations or ill effects. Plus, with as many times as i had the tires flipped on the rims throughout each season to maximize tread life, it would get too expensive to have them balanced each time.

For track events on real race tracks, i would probably balance them since you'd be running at higher speeds for longer amounts of time.