Also looking at the power difference at the two different rpms, I calculate 160 at 5200 and 180HP at 6000, whether that is  accurate absolute power it shows a possible 12.5% increase.  The disc area carved out by the 74" prop is approx 18% more although its speed is approx 13% less.  So I think more area, more HP and more pitch appears to add up in my case to considerably better low speed performance.  The top speed appears not to have changed, still a shade under 200 MPH around 197MPH for both props.