OD is the 4th gear. if you turn it off, then you just have a 3spd tranny with the same shift points as before, it just wont go into 4th gear.
OD puts you in a gear that lowers your rpms and improves gas milage. it is very beneficial on the highway. in the city with the stop and go traffic, it can still help if you catch some green lights or you know its in 4th. anytime youre in 4th gear (od) then your engine is not working as hard as it is in the other gears. i believe its a 1:1 ratio at that point from engine to tranny.
bottom line: leave it on all the time. taking it off does not improve acceleration or 1/4 times at all. taking it off however does decrease milage and makes the tranny work harder at the top of 3rd gear
**I have sigs turned off, so if you're asking for advice based on your mods, I can't see them**