Like Scott said, in AX it's not the car that makes you fast, only the driver. A skilled driver in a car right off the showroom floor will badly spank a novice in the best prepared car (seen it done with rentals!). If you want to mod your car go ahead, and then AX it in whatever class it falls into for fun and experience. You'll spend a lot more time in it on the street than on an AX course. Once you get experience AXing, you'll know if you want to compete seriously and be familiar with the classes and prep rules, so you'll know what to do to prepare a serious AX car.
No matter what tho, I'd forget about replacing the rear glass!