I drive most of the time on manual, automatic can be annoying. Still I play automatic sometimes and it really doesn't make a massive difference in time. For this time trial I use the paddles. It's gonna take you some time to get used to it, you'll actually be slower but in the long run you'll do better.
Just improved on the stock bringing my best time to 1'49.459
http://eu.gran-turismo.com/es/academy/2010/ranking/?board=0&area=69