Faiz,

To drive fast in a straight line is to leave it in "D" and let the auto do the work. Like Viperx said the gear change program is optimized so doing it manually would not be quicker.

Tiptronic is good for up and down the mountain roads where there's alot of corners and
engine brake for going downhills.

When you cornering fast, you'll need to stay in the same gear (eg. 2nd gear) from entering to exiting the corner without upsetting the balance of the car. Sudden kickdown during cornering could be dangerous.
It's always good to use a bit of engine brake downhill to help slow down the car.