After going from last to first in 10-laps on Pro mode at Michigian, I decided I HAD to try something to make it more challenging.
What works for me is to use your fifth gear setting as sort of a "rev limiter." Lower the top speed to a level where you can pass if you run a clean lap, but not blow by the competition.
I set it a 224-5 and got in six car pack race for the win... and couldn't get by, because I could get a run, but cars on the inside line would crowd me. Good fun, I'm sure I could get a win sometimes, but have to drive my butt off to get it!