Time to 'get nautical' and set your GPS for knots!
If you can't achieve rated RPM it's likely you're 'overpropped' and repitching may get you back up the the max revs. It may not give you any more speed.
Using GPS for speed can be deceiving, esp if any current is a possibility. Around here if you time things wrong you could be doing hullspeed through the water and a GPS will show close to zero.