Yes Steve, you do have to pay to have old motor removed and new installed, it is pricey! I whish I could be more DIY kind of guy, would of saved a lot of cost.
I have the new Yamaha F70, it is the same 996 cc motor as the 50/60 but tricked out to 70hp with 2.33 gear reduction ratio, lots of torque.
After reading a lot of experiences on other Mac sites I decided that a 14 X 11 prop would be a good choice, turns out it was. The recommended RPM range is 53-6300 rpm and I was getting the upper range 62-6300 @ WOT. So I decided to get a spare prop just in case and purchased a 13 5/8 X 13 prop. I took it for a test run thinking I would lose 200 rpm per extra inch of pitch so would lose about 400 rpm and get 5800 rpm but also thought I would gain maybe 100 rpm for the drop to 13 5/8 diameter. In the end I expected to get maybe 5900 rpm, middle of recommended range, and more speed.
Well, I was surprised to find I lost a whopping 1000 rpm, I only got 52-5300 rpm @ WOT, empty ballast. Hard to comprehend and I cannot explain it but I did get 22mph compared to 19mph on the 14×11 @ WOT, cannot explain that either, suffice to say those were the recorded results I had. I since switched back to the 14×11 and keep the 13 as a spare since I seldom use WOT. Not sure if the higher end of range is better than low end of range but we tend to load the boats heavy so I chose the higher end.
I have only ever heard good things about Yamaha so that is why I chose it and am satisfied with my decision. Still the best power/weight ratio out there! 253 lbs! E-tech 60 is 239lbs Honda 50 about 215 lbs and Suzi 70 from BWY was a whopping 370 lbs in its day!
Many boats are getting older now and the engines are getting tired so re-power decisions are increasing. I believe 70 hp is the minimum hp for the macs while others are choosing 90 hp especially with the X model. The M should be just fine with 70 hp. Gotta love the big 14″ prop you can swing.
PS I also believe the increase in hp increases the resale value, and makes the boat more desirable to potential buyers.