Pitch and length are important. My Y6 flew fine with Graupner 11x5 props. I actually got more flight time when compared with the 10 inch APC props. I had 11x5 inch Graupners on top and bottom motors and noticed that in order to maintain altitude that I had to keep the throttle a few notches above the halfway mark. I replaced the bottom props with Graupner 12x6 and my ship is rock solid. No yaw issues, plenty of lift and decent flight time considering my craft is almost 7 pounds.
The bottoms motors/props couldn't generate enough lift in already disturbed air. Changing the length and pitch really made a worth while difference in flight characteristics.