First... How are you measuring the speed? The speedo on these things are almost never right. If you are getting 55, an he is getting 60... you are in the error range of the factory speedo. If you are side by side, and hes pulling an extra 5 MPH... there can be a bunch of things.
1) his pipe water may be plugged up. A dry pipe is faster on the top... but if not set up right, can cause problems
2) his impeller isn't the same as yours. (has it been changed?)
3) your impeller or wear ring have some wear.
4) you have a slight drive shaft seal leak, or pump shoe leak. (do you feel any cavataion when you hit the throttle?)
5) your fresh engine isn't giving you that last few RPM yet.
6) Old fuel, or fouled plugs??
Basically... if you are getting into the mid 50's verified by a GPS... then you are running right for a ski with some time on it. If you want to squeeze out that extra 5mph... you need to go over the drive, and correct any little imperfection.
Hope that helps.