And one emailer has mentioned when they used the engine on a mazda
powered a/c they were limited to 170 mph with the IVO but got 213 mph
with a cruise wood prop.

One of the parameters of a propeller is the "helix angle." To give a
muddled explanation, think of the propeller as "screwing" its way
through the air. At really high forward speeds, the propeller needs a
lot of twist to maintain a constant angle of attack along the entire
blade. At lower forward speeds, it needs much less twist.

If you have a prop with a low speed helix angle and start trying to take
it too fast, not all parts of the prop will be pulling their weight
equally. When you're checking your potential props, ask about helix
angle. The 170 vs 213 MPH sounds like that could be a big part of it.

For example, on light twins, apparently the prop blades have helix
angles set for about 100 - 120 MPH. This is so they will have some
single engine climb performance. A single, on the other hand, might have
the helix angle selected for cruise.

Ed Wischmeyer