A 500cfm 2 barrel only flows 353cfm when converted to the scale 4 barrel carbs are tested at.
I would definitely recommend a mechanical secondary 4 barrel carb for the precise throttle control needed for autocross. I would stay on the smaller side size wise for crisper response.
A vacuum secondary carb should be avoided because it has a delay when the secondary's open and close. This delay will cause the car to be harder to control in a drift or slide when you need to lift and get back on the throttle quickly which will hurt your times and your ability to control the car under power. You want the carb to immediately do what your foot tells it to do.
