I don't know how accurate you speedo is but I assume it is not since you are talking RPM. My simple guess is that you were going 1 or 2 mph faster with the 13x12 prop because that will lengthen the wake. Try slowing down more and I bet your old wake will return. The RPM differences are due to the different prop (sounds like you know this). We set our perfect pass at 11.4 MPH for surfing and less than 1 MPH speed difference makes a huge difference in the wake. I just think you need to find the proper RPM to get the same boat speed as the old prop. It is possible the new prop will not hold the same speed you went with the old prop since the RPM is much lower with the new prop.
2009 Supra Launch 24 -Gravity Games