habsr1 - something is not set up right.
My 2008 Z9 with 250 Pro XS with a full tournament load, full gas tanks (67 gallons) and two guys runs 72 mph GPS as set up coming out of the factory with a 26" Tempest prop, turning 5800 RPM. I'm still not out of the 3 hour break-in period so I'm not pushing it too much yet nor have I made any adjustments to the jackplate. Once I get out of the break-in period, tune the boat for speed and lighten the load a bit I should hit 75+ without much issue.
When you say you're only getting 68mph, what RPM are you running?
What pitch & make prop?
What year boat?
Fusion hull or standard handlaid hull?