I dont think a Fury 3 will work as well as a tempest on that boat. I also believe that many would be happier with a four blade on that boat. If your numbers are accurate that is upward of 20% prop slip. Indicating a serious proplem with the prop or a issue with the running pad.
How was RPM read, analog or smartcraft. Was speed read on GPS and was it verified by another GPS as well. I found out first hand that a GPS can be wrong. I had a Lowrance unit that was off by 10 MPH. I performed a master reset on the unit and that corrected it.