It is simply a manual method to force (well, not force, the TCU will not allow you to change gears if changing gears will overspeed the engine) a gear change.

I don't think ANY of us can predict the exact rpm change between gears, there are five gears and no one has that memorized. In any event, you can't overspeed the engine, as I said, the TCU will not allow a shift that is harmful to the engine. If you get good with the manual + and -, you MIGHT get the car to move quicker - I suppose.

I always used it to downshift as I slow, I've had that habit my whole life - I use the brakes less than the average person.