I have been flying a Pico cub as a trainer and am using a 9x5 prop on a 5.1 geared 400. I use a 9.6v 600mh and like the performance but only get about 8 min out of it. I also use a 7.2v/600mah and it is more flight time. Will bringing down the prop size and pitch to say about a 8.4 give me more flight time?

Anything you do to reduce the current draw will increase flight time. Keep in mind that it will also reduce performance.

Reducing voltage reduces the current being drawn by the motor at full throttle. That's why you see longer flight times.
Using a smaller prop will also reduce current draw. The only thing I'd worry about is reducing the prop size too far, and not having enough power to fly the airplane.