How much gain with lower gears?

So just to keep it simple I'm running a mild 408 in my Sport. It's 90% a street car that will see some weekend bracket racing. I'm currently running 323's in a 8.75 suregrip that's perfect for the street. The center is older and I was planing a rebuild soon. If I went to a 3.55 or at the lowest 3.73, how much could I possibly shave of my et.? I have not run the car on the track yet, just wondering if it would be a dramatic gain or if It would just be a tenth or two. Thanks