what RPM's are you getting now at wide open throttle? Got to thinking, engine height could affect speed, but not by 10 mph. A jackplate is an investment, but would allow you to tweak the rig with the right prop and improve driveability and will typically get you 1-2 mph increase.
The plugs shouldn't be affecting the speed unless they aren't firing and you aren't running on all cylinders. call the Ranger factory and get ahold of the rigging department, they can tell you what prop they rigged that model boat with for a 125hp typically and that is a great place to start. Until you get close to the right prop, hard to tell if engine is running right. Good Luck.