I have the H-20 Bass boat with a 200 on it. I can tell you that a 115 will probably not reach a real 50 mph, much less cruise at that speed. If you could reach 50 mph you would be burning a lot more fuel than if you were running a 150.
I live on a very busy river and see these boats all of the time. You should ask your dealer to give you a demo ride or to get one of his customers to let you ride. A 115 is a great motor on a smaller boat, but I think you will be really disappointed if you put that on a 20' boat. Will the 115 push your boat? Sure it will, but when you invest as much as these boats cost...better get the right motor or you'll never be satisfied.
While talking about "how fast" I was running my boat yesterday and was noticing that my speedometer was showing about five to six mph faster than the gps. I watched the speedometer as I increased speed and at a real gps speed of 63, it was showing 70 mph.