I'd suggest try and fly, with a measured course and a stopwatch (or a radar gun). Which prop will be the fastest will depend on your engine's horsepower curve, your airplane drag, and many etc.'s. I suspect a 10 x 6 or 7 will be faster than your 11 inch props. Speed and racing people spend a lot of time trying props. As said, not all 10 x 6's are created equal.