He's right. I know a guy that has on and he don't like pushing it past 45 mph. It is a real dog, especially on hills. But he bought it for the same reason that you are thinking, cool to have at a show. But it don't compare to his shovel. That 45 is a small motor. Those flatheads are only about 7 to 1 on compression ratios. So it will run on just about anything. Even if you put a 80" flathead motor in it, it would not want to run at 60 steady. Friend of mine has a 1947 80" flathead bike and it is screaming at 60. He rides it on the back roads putting around 20-40. Real cool machine.
Buy it and take the long slow ride to the shows...