Like all 2-stroke outboards, there is no problem operating them at full throttle for long periods. You will find however, if you back off the throttle just a bit from top end you will greatly increase your gas mileage.
Given the motor is an 89, make sure you always have a strong flow of water comming out of the “pee hole”. If it’s been more than 3-4 seasons of use since the impeller has been changed, it may be time.
34 MPH seems reasonable for top end on a 17 footer with a 60hp. With a lot of trial and error testing different props, you may gain 1-3 mph. Probably not worth the effort.
J.