In my opinion, the numbers you describe for a Powerdrive 55 and that boat are about dead on. 1.5 SOG on the GPS is about as fast as you will probably get for that boat and that motor. Only thing you might be able to do to get a little more out of her is to raise the main motor out of the water (if you have not done so already).
One thing I have noticed however. With the powerdrive 55 and a standard pedel or the "Co-Pilot" system, your speed can only be increased in full increments up to the number 5. With the I-pilot installed the the PD 55, it goes in 1/2 increments to the number 10. I doubt it would give you much more on the top end but it does allow for fine mid range speed tuning and lower speeds. Not that it will help me much.