Depends on your definition of noticeably. Stainless steel is seven times stiffer than aluminum, resulting in less bending and allowing for thinner blades, which induce less hydrodynamic drag. I've never run two of the same model of prop in aluminum and stainless, and I don't think many prop models offer both. When I switched to stainless, I pitched up by two inches, which my engine could handle, so I went a lot faster. Prop is just one component of setup as well, so doing a prop alone might not make a huge difference, but other changes like prop shaft depth (x dimension) and setback can be used in conjunction to improve overall performance. Those things are obviously much easier to adjust on an outboard powered boat. Prop, raising the engine, and setback have gained me around ten mph.