How much gain with lower gears?

A chassis dyno will only give sort of usable numbers. Get the car on the track. Get it consistent. Then change the shift points by 500 rpm. If it goers faster, it could probably use deeper gears provided it pulls cleanly in high gear past the stripe. If it doesn;t change much, don't plan on seeing any major night and day change. Because more gear will change the consistency. The convertor will change, and hit the suspension and tires a bit differently.