The basics are:
More transmission at front wheels = better steering.
More transmission at rear wheels = better acceleration (to a certain point).
The more transmission you put to your front wheels the more they can actively pull your car in a direction your front wheels facing.
The more transmission you put to your rear wheels the more your car is gonna be pushed in a straight line forward (direction your rear wheels facing).
There are no settings which could be called "best" it strongly depends on preferences of the dirver and track.
If you like to go full throttel during corners (which you should with a 4wd if possible) you are better with more front transmission. If you are racing a track with many smooth long corners you should have more rear transmission.
To be honest i don`t think it is worth to change all/most to rear transmission, top speed won`t change and your acceleration won`t benefit alot from doing so. You`ll only notice getting pushed from the line during corners with full throttel.