Basic driving skill : Brake late, Accelerate early smooth is the key.
For example turn 1 (at the end of long straight like Pattaya Pira)
100% BRAKE before turn (as late as posible)
start TURNING and at the same time DOWN SHIFT and start ACCELERATION thru the apex.
To answer the 1st question assume for auto-x.
So if you get all the down shift done in the turn before the apex. You can get out of the turn quicker make it faster. This is happen verry quickly.
Like O'Pern said your question is not clear but I gonna guess for 2nd question.
If you down shift use gear to slow down the car (Chain Gear) the engine might hit the redline and BOOM. Piston go thru the block no good :'(. Always use brake to slow down the car.
I hope I answer that right ???