The touch up paint is time consuming, but if done properly can produce some pretty good results.
Different manufacturers label "overdrive" differently. Some manufacturers simply have the "lock up" torque converter as their overdrive. This means that you have no losses from the torque converter at this point. The torque converter transfers power to the wheels basically by spinning a drum that have fan blades on the inside. As the fan blades spin and catch the transmission fluid, that fluid catches another set of blades and turns the transmission. A very simplified explanation, but you can see how you would lose a lot of power in this transfer.
Now, for the manufacturers that actually have a real "overdrive" it is different. An overdrive gear means that for every revolution that the engine makes, the transmission will make MORE than one revolution. This is because of the gear ratios. So, basically you are getting more turns out of your wheels than the engine is providing. If your engine is making less revolutions, you are using less fuel.
I hope that helps. Now with maufacturers having 5, 6, and 7 speed transmissions you can have 2 or even 3 overdrive gears.
Tom