the rear is really a matter of preference-they don't do a whole lot to actually stop the car.the main advantage to discs is just ease of service,and it's easier to check pad wear.if you can get a good deal on the conversion i'd do it.also don't forget the proper proportioning valve.
what front pads are you running?the compound is a huge factor in performance-i've had 9.1" front brake cars with agressive pads that stop one million times better than a 10.3" car with mild pads.i'm partial to Carbotech.they have a great compound for autocross and one for track use that are both streetable.as light as your car is,i think you could get away with a good front pad and be satisfied.if you do decide to go bigger,i think the 10.3" fronts off a 90-91 Civic Sedan EX are the easiest swap.that car also has a 15/16" master cylinder that will greatly improve pedal feel and power with stock or bigger brakes.

my hatchback has 94-01 front and rear brakes.i'm running the above-mentioned 15/16" MC,Super Blue fluid,Carbotech XP9 front pads and Autozone replacement rear pads.i have no problems at all with fade or power during 40 minute sessions,including two stops per lap from 130MPH down to 50MPH.i went to the mild rear compound because the car was a little unstable for my liking under braking with the heavy rear spring/big rear bar setup.for autocross you may want a more agressive rear pad to help with rotation.