Here's a thread in the Tachometer Forum. Some posted RPM numbers with different props. I personally think some of these numbers are too high. I wouldn't crank a Ryobi that hard. Thats why I like the 18-6 prop. Plenty of thrust--good speed, and keeps the engine in the RPM band is was designed for.
http://www.rcuniverse.com/forum/m_32.../tm.htm#328508