We've been down this road MANY times.
Taking 13.5K rpm, and being generous, you could say it unloads to 15K in the air. With 8" pitch that comes out to 113 mph. In a big ol' dive you can usually do about 10% over the prop speed or in this case 124 mph. Add a 15 knot tail wind gust and you
might see 140.
You are correct to be sceptical!
If you've got time you might read here >>
http://www.rcuniverse.com/forum/m_77...tm.htm#7781638
Bill