For quietness, I believe it is important to over power the plane somewhat. For example, a sixty running at 2/3 throttle makes about the same power as a forty running wide open but makes less noise and probably uses less fuel as well. I did something like that with a Kadet Senorita. I removed the .25 FP and replaced it with a .40 FP. The larger engine didn't have to work as hard to fly the plane so it ran lower rpm most of the time with the throttle only about half open. As a result, the flights were actually longer on the same fuel tank and a neighbor of mine actually did mistake it for an electric plane. However, in the hands of some kid who runs the engine at full throttle for the whole flight as he bores holes in the sky, a bigger engine will indeed be louder and thirstier.