I would not be concerned when using a glass filled prop as long as it is balanced. According to Thrusthp calculator an 8x6 and a 7x10 would have close to the same load. The seven inch would be faster IF the airframe is small/clean enough. You could also take an 8x7 or 8x8 and cut it down to about 7.5". I would use a prop that would unload to atleast 18,000 in the air or around 17,000 rpm on the ground.