don't know if anyone said this ( did not read every post ) , but I have had very good luck with Graupner 3 blade props.

you should be able to go to a 9x7 and get away with it, and I do believe we loss a little performance with the three blade props. I was told it is due to dirty air, in other words the blades are soo close to each other they don't get a good " bite ".

sticks