My experience with the OD switch is it's a better drive with the OD on. With the OD off, 3rd gear also has a lock-up that hinders shift points, so when accelerating it wants to grab into 3rd gear early and is stubborn to downshift. With OD on, there isn't a lock-up in 3rd gear, so I get better shift action under acceleration, and it seems to downshift easier when I get on the gas.
For a while I ran the mod that automatically switched off OD when the shifter passed through reverse, but ended up pulling the ground on the relay to run with OD on.
In my case, the factory design takes the win. I am running 35s with 4.88 gears and a 4.6 stroker tho, so that might play into the equation. So much for factory, I guess
: )