i have a 21 foot Ultra with the same set up with a three blade23 mirage plus I was getting 70 on my speedo but i wouldnt let it stay wide open long enough to see how fast it would go. Now that it is broke in I have switched props to a 4 blade 21 from Valley prop to improve my hole shot I now have had it up to 68 on the speedo at 4400 but didnt go all the way to 4600 which is optimal for this motor ,Also the speed on the dash is not all that acurate so maybe you are doing better than you think GOOD LUCK