Cranky after doing some research wanting to do something similar to you I found out that just because a motor has a higher lb rating doesnt mean it will go faster, from what I remember reading the rpm prop size etc have effect on the actual speed of the craft. So if you were to increase the prop size for instance you could probably get a much higher speed out of the same lb motor.
One of the articles from min kota that i read actually stated that their motors are made more for acceleration than speed, and im sure that could be counteracted by a bigger prop.
Hope this helps, if it does in anyway please let me know as i intend on doing something very similar in the future.