How much gain with lower gears?

Based on the lack of info given, its impossible to say. Convertor, cam, carb, compression, etc.
Its extremely unlikely you will see enough difference to like the change.
Those saying 1/2 a second..... haha. Cant see any way just a gear change by itself willl make that kind of change.
I have never seen even a tenth, ever, on any gear swap i have ever made.