I see on another post that you've already decided on a 55lb TM so that decision is out of the way.
I will add to this discussion that people often say that a 30lb TM moves a canoe nicely so you don't need any more thrust. BUT, while a 30 will indeed move the boat OK in still water and no wind - it's a different thing when you're trying to cut through slop or get hit with a sudden thunderstorm with 30mph winds and you're headed into the wind to get back to your launch point.
I started with a 45lb TM for my 12ft, 34 pound canoe, and still use that for smaller lakes or when I am relatively certain that weather will not be a factor. For larger lakes or when I'm expecting bad weather, I run a 55lb TM which is as big as you can generally go on 12 volts.
Regarding your questions about speed, IndianaOutdoors was pointing you to one of the limiting factors. On a displacement hull like a canoe (in other words, a hull that will not generally go "on-plane"), there is a factor called the hull speed. That is the theoretical maximum speed that the hull can make without a CONSIDERABLE increase in power. Just Google "hull speed" and you should find some light reading. The higher the length-to-beam ratio, the higher the theoretical hull speed. That is why long, narrow racing shells, many kayaks, and some long/narrow canoes have considerably higher hull speeds that some of the run-of-the-mill fishing canoes with wide beams.. My boat is only 12 ft (11' 6" actually) with a portly 38" beam and it's theoretical hull speed is probably very low.
The other limiting factor in using a TM as your primary propulsion is that they are not made for speed, and the pitch of the propeller is set for lower speeds and for handling wind and cutting through slop. Whether you have a 30lb TM, or a 55lb TM, if both motors have the same prop pitch (and same diameter), and both have about the same maximum RPM, they are going to go about the same speed. The difference is that the 55 has enough thrust (power) to achieve that maximum RPM while going into the wind, but the 30 may not and will thus not be able to achieve the same speeds as the 55 in those conditions.
As A-Jay said above, running TMs on canoes is not about speed; it's about not having to paddle, to be able to fish while underway, to have power to hold in the wind, etc. If you want speed on a canoe, you need to consider a gas engine.