My rig will have (2) 55# on the rear, and (1)45# bow mount. I expect to get about 4-4.5mph out of it. My 13ftr had a 55# on the front, and a 55# on the back, and would run about 3.8mph. If I turned off the back motor, it would run about 3.2mph. Point being, it's alot of money and weight for an extra one on the rear.
Breaking 5mph on trolling motors is no easy feat. Most (if not all) will need at least (2) 24V motors to do so. Boats have a max hull speed unelss on plane, which TM won't do. One 80# motor will push the boat better than 2 40# motors, because there is less drag. You'll also get longer runtime out of the one 80# motor than the 2 40#. You can look up the hull speed calculator and see what your max hull speed is. It makes the decision a little clearer if you realize you're .5mph from max hull speed and debating on a second or third motor. If you're on a tight budget, I'd carry two 55# motors (one up front, and one in the back, with 3 batteries) The third battery will run electronics and can be used in an emergency. I've fished alot of 8hr tournaments with that setup. I just don't expect to get to any spots first. IMO, there is no such thing as too much TM. The stronger motor does better up front. It seems they pull better than push. I know someone who had (3) 24V motors on his boat. He sold one of the three, and lost .5 mph. Price 2 batteries, and the motor, and the .5mph doesn't look so good. Not uncommon here for guys to run 3-4 motors, but it's because they are fishing tournaments, and they feel the .5mph is worth it.