when you manually shift the car it wont add to the car at all, it wont make any more power. Basically all you are doing is telling the car where to shift at.
With a good tune for the car leaving it in drive will yield better performance results as the car can then shift better than you can manually.
I used to want to do this with my old 97 V6... but it didnt help
I ended up realizing I just really wanted a manual transmission and finally got one in my next car lol.
Some say manually shifting will hurt the car, not sure why really~ and overdrive is just another gear, nothing special so leaving it off just means you wont shift into 4th gear.