I had a few 17.5's but managed to get over excited and run wide coming out of the left handers onto the straight. The quick right to get onto those continuous left handers is the tricky part for me and was most certainly my undoing in the NASCAR TT. I know how to do it, its just getting it done is the hard part.
Smooth inputs with the wheel and throttle are definitely needed. I am using 3rd gear for the first corner and for the start of the final chicane. It helps get the car turned in and set. And I have a magic final sector that's 2 tenths faster than anything else I've managed using 3rd gear there. Finding the right braking and turn in points for that turn is really tricky.