How much gain with lower gears?

As much as a half second. My car goes 12.20's in 4th gear. 11.70's in 3rd gear.

What you want is a gear that will put you through the traps at about peak hp RPM. If you don't know what RPM that is you are pissing in the wind.

Also keep in mind where you want to be at cruise RPM. Balancing street and strip without an OD gear takes some conservative thinking.

3.55's will put you at about 2500@ 60mph with a 28" typical tire. 3.73's will put you at about 2800.

I'd be running 3.55's if I were you. Or a GVOD and some big ole gears......